XL: GetObject caching implemented for XL. (#2017)

The object cache implementation is XL cache, which defaults
to 8GB worth of read cache. Currently GetObject() transparently
writes to this cache upon first client read and then subsequently
serves reads from the same cache.

Currently expiration is not implemented.

globals.go

@@ -43,6 +43,8 @@ var (
 	// Maximum connections handled per
 	// server, defaults to 0 (unlimited).
 	globalMaxConn = 0
+	// Maximum cache size.
+	globalMaxCacheSize = uint64(maxCacheSize)
 	// Add new variable global values here.
 )