Prevent unnecessary verification of parity blocks while reading (#4683)

* Prevent unnecessary verification of parity blocks while reading erasure
  coded file.
* Update klauspost/reedsolomon and just only reconstruct data blocks while
  reading (prevent unnecessary parity block reconstruction)
* Remove Verification of (all) reconstructed Data and Parity blocks since
  in our case we are protected by bit rot protection. And even if the
  verification would fail (essentially impossible) there is no way to
  definitively say whether the data is still correct or not, so this call
  make no sense for our use case.

vendor/github.com/klauspost/reedsolomon/README.md

@@ -81,6 +81,17 @@ To indicate missing data, you set the shard to nil before calling `Reconstruct()
 ```
 The missing data and parity shards will be recreated. If more than 3 shards are missing, the reconstruction will fail.
 
+If you are only interested in the data shards (for reading purposes) you can call `ReconstructData()`:
+
+```Go
+    // Delete two data shards
+    data[3] = nil
+    data[7] = nil
+    
+    // Reconstruct just the missing data shards
+    err := enc.ReconstructData(data)
+```
+
 So to sum up reconstruction:
 * The number of data/parity shards must match the numbers used for encoding.
 * The order of shards must be the same as used when encoding.
@@ -101,7 +112,7 @@ You might have a large slice of data. To help you split this, there are some hel
 ```
 This will split the file into the number of data shards set when creating the encoder and create empty parity shards. 
 
-An important thing to note is that you have to *keep track of the exact input size*. If the size of the input isn't diviable by the number of data shards, extra zeros will be inserted in the last shard.
+An important thing to note is that you have to *keep track of the exact input size*. If the size of the input isn't divisible by the number of data shards, extra zeros will be inserted in the last shard.
 
 To join a data set, use the `Join()` function, which will join the shards and write it to the `io.Writer` you supply: 
 ```Go
@@ -153,7 +164,7 @@ This also means that you can divide big input up into smaller blocks, and do rec
 
 # Streaming API
 
-There has been added a fully streaming API, to help perform fully streaming operations, which enables you to do the same operations, but on streams. To use the stream API, use [`NewStream`](https://godoc.org/github.com/klauspost/reedsolomon#NewStream) function to create the encoding/decoding interfaces. You can use [`NewStreamC`](https://godoc.org/github.com/klauspost/reedsolomon#NewStreamC) to ready an interface that reads/writes concurrently from the streams.
+There has been added support for a streaming API, to help perform fully streaming operations, which enables you to do the same operations, but on streams. To use the stream API, use [`NewStream`](https://godoc.org/github.com/klauspost/reedsolomon#NewStream) function to create the encoding/decoding interfaces. You can use [`NewStreamC`](https://godoc.org/github.com/klauspost/reedsolomon#NewStreamC) to ready an interface that reads/writes concurrently from the streams.
 
 Input is delivered as `[]io.Reader`, output as `[]io.Writer`, and functionality corresponds to the in-memory API. Each stream must supply the same amount of data, similar to how each slice must be similar size with the in-memory API. 
 If an error occurs in relation to a stream, a [`StreamReadError`](https://godoc.org/github.com/klauspost/reedsolomon#StreamReadError) or [`StreamWriteError`](https://godoc.org/github.com/klauspost/reedsolomon#StreamWriteError) will help you determine which stream was the offender.
@@ -162,6 +173,18 @@ There is no buffering or timeouts/retry specified. If you want to add that, you
 
 For complete examples of a streaming encoder and decoder see the [examples folder](https://github.com/klauspost/reedsolomon/tree/master/examples).
 
+#Advanced Options
+
+You can modify internal options which affects how jobs are split between and processed by goroutines.
+
+To create options, use the WithXXX functions. You can supply options to `New`, `NewStream` and `NewStreamC`. If no Options are supplied, default options are used.
+
+Example of how to supply options:
+
+ ```Go
+     enc, err := reedsolomon.New(10, 3, WithMaxGoroutines(25))
+ ```
+
 
 # Performance
 Performance depends mainly on the number of parity shards. In rough terms, doubling the number of parity shards will double the encoding time.
@@ -186,6 +209,18 @@ Example of performance scaling on Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz - 4 ph
 | 4       | 3179,33 | 235%  |
 | 8       | 4346,18 | 321%  |
 
+Benchmarking `Reconstruct()` followed by a `Verify()` (=`all`) versus just calling `ReconstructData()` (=`data`) gives the following result:
+```
+benchmark                            all MB/s     data MB/s    speedup
+BenchmarkReconstruct10x2x10000-8     2011.67      10530.10     5.23x
+BenchmarkReconstruct50x5x50000-8     4585.41      14301.60     3.12x
+BenchmarkReconstruct10x2x1M-8        8081.15      28216.41     3.49x
+BenchmarkReconstruct5x2x1M-8         5780.07      28015.37     4.85x
+BenchmarkReconstruct10x4x1M-8        4352.56      14367.61     3.30x
+BenchmarkReconstruct50x20x1M-8       1364.35      4189.79      3.07x
+BenchmarkReconstruct10x4x16M-8       1484.35      5779.53      3.89x
+```
+
 # asm2plan9s
 
 [asm2plan9s](https://github.com/fwessels/asm2plan9s) is used for assembling the AVX2 instructions into their BYTE/WORD/LONG equivalents.