Add disk based edge caching support. (#5182)

This PR adds disk based edge caching support for minio server.

Cache settings can be configured in config.json to take list of disk drives,
cache expiry in days and file patterns to exclude from cache or via environment
variables MINIO_CACHE_DRIVES, MINIO_CACHE_EXCLUDE and MINIO_CACHE_EXPIRY

Design assumes that Atime support is enabled and the list of cache drives is
fixed.
 - Objects are cached on both GET and PUT/POST operations.
 - Expiry is used as hint to evict older entries from cache, or if 80% of cache
   capacity is filled.
 - When object storage backend is down, GET, LIST and HEAD operations fetch
   object seamlessly from cache.

Current Limitations
 - Bucket policies are not cached, so anonymous operations are not supported in
   offline mode.
 - Objects are distributed using deterministic hashing among list of cache
   drives specified.If one or more drives go offline, or cache drive
   configuration is altered - performance could degrade to linear lookup.

Fixes #4026

docs/config/README.md

@@ -92,6 +92,13 @@ minio server /data
 
 By default, parity for objects with standard storage class is set to `N/2`, and parity for objects with reduced redundancy storage class objects is set to `2`. Read more about storage class support in Minio server [here](https://github.com/minio/minio/blob/master/docs/erasure/storage-class/README.md).
 
+### Cache
+|Field|Type|Description|
+|:---|:---|:---|
+|``drives``| _[]string_ | List of drives|
+|``exclude`` | _[]string_ | List of wildcard patterns for prefixes to exclude from cache |
+|``expiry`` | _int_ | Days to cache expiry |
+
 #### Notify
 |Field|Type|Description|
 |:---|:---|:---|