migrate all bucket metadata into a single file (#9586)

this is a major overhaul by migrating off all
bucket metadata related configs into a single
object '.metadata.bin' this allows us for faster
bootups across 1000's of buckets and as well
as keeps the code simple enough for future
work and additions.

Additionally also fixes #9396, #9394

cmd/xl-v1.go

@@ -51,6 +51,8 @@ type partialUpload struct {
 
 // xlObjects - Implements XL object layer.
 type xlObjects struct {
+	GatewayUnsupported
+
 	// getDisks returns list of storageAPIs.
 	getDisks func() []StorageAPI