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

cmd/gateway/sia/gateway-sia.go

@@ -73,6 +73,11 @@ ENVIRONMENT VARIABLES: (Default values in parenthesis)
   BROWSER:
      MINIO_BROWSER: To disable web browser access, set this value to "off".
 
+  CACHE:
+     MINIO_CACHE_DRIVES: List of cache drives delimited by ";"
+     MINIO_CACHE_EXCLUDE: List of cache exclusion patterns delimited by ";"
+     MINIO_CACHE_EXPIRY: Cache expiry duration in days
+
   UPDATE:
      MINIO_UPDATE: To turn off in-place upgrades, set this value to "off".
 
@@ -85,6 +90,12 @@ ENVIRONMENT VARIABLES: (Default values in parenthesis)
 EXAMPLES:
   1. Start minio gateway server for Sia backend.
       $ {{.HelpName}}
+
+  2. Start minio gateway server for Sia backend with edge caching enabled.
+      $ export MINIO_CACHE_DRIVES="/home/drive1;/home/drive2;/home/drive3;/home/drive4"
+      $ export MINIO_CACHE_EXCLUDE="bucket1/*;*.png"
+      $ export MINIO_CACHE_EXPIRY=40
+      $ {{.HelpName}}
 `
 
 	minio.RegisterGatewayCommand(cli.Command{