Refresh was doing a linear scan of all locked resources. This was adding
up to significant delays in locking on high load systems with long
running requests.
Add a secondary index for O(log(n)) UID -> resource lookups.
Multiple resources are stored in consecutive strings.
Bonus fixes:
* On multiple Unlock entries unlock the write locks we can.
* Fix `expireOldLocks` skipping checks on entry after expiring one.
* Return fast on canTakeUnlock/canTakeLock.
* Prealloc some places.
it would seem like using `bufio.Scan()` is very
slow for heavy concurrent I/O, ie. when r.Body
is slow , instead use a proper
binary exchange format, to marshal and unmarshal
the LockArgs datastructure in a cleaner way.
this PR increases performance of the locking
sub-system for tiny repeated read lock requests
on same object.
```
BenchmarkLockArgs
BenchmarkLockArgs-4 6417609 185.7 ns/op 56 B/op 2 allocs/op
BenchmarkLockArgsOld
BenchmarkLockArgsOld-4 1187368 1015 ns/op 4096 B/op 1 allocs/op
```
