Add bucket metadata state client/handler (Fixes #3022) (#3152)

- Adds an interface to update in-memory bucket metadata state called
  BucketMetaState - this interface has functions to:
     - update bucket notification configuration,
     - bucket listener configuration,
     - bucket policy configuration, and
     - send bucket event

- This interface is implemented by `localBMS` a type for manipulating
  local node in-memory bucket metadata, and by `remoteBMS` a type for
  manipulating remote node in-memory bucket metadata.

- The remote node interface, makes an RPC call, but the local node
  interface does not - it updates in-memory bucket state directly.

- Rename mkPeersFromEndpoints to makeS3Peers and refactored it.

- Use arrayslice instead of map in s3Peers struct

- `s3Peers.SendUpdate` now receives an arrayslice of peer indexes to
  send the request to, with a special nil value slice indicating that
  all peers should be sent the update.

- `s3Peers.SendUpdate` now returns an arrayslice of errors, representing
  errors from peers when sending an update. The array positions
  correspond to peer array s3Peers.peers

Improve globalS3Peers:

- Make isDistXL a global `globalIsDistXL` and remove from s3Peers

- Make globalS3Peers an array of (address, bucket-meta-state) pairs.

- Fix code and tests.

cmd/event-notifier_test.go

@@ -291,7 +291,7 @@ func TestInitEventNotifier(t *testing.T) {
 	// needed to load listener config from disk for testing (in
 	// single peer mode, the listener config is ingored, but here
 	// we want to test the loading from disk too.)
-	globalS3Peers.isDistXL = true
+	globalIsDistXL = true
 
 	// test event notifier init
 	if err := initEventNotifier(obj); err != nil {
@@ -366,7 +366,7 @@ func TestListenBucketNotification(t *testing.T) {
 	// needed to load listener config from disk for testing (in
 	// single peer mode, the listener config is ingored, but here
 	// we want to test the loading from disk too.)
-	globalS3Peers.isDistXL = true
+	globalIsDistXL = true
 
 	// Init event notifier
 	if err := initEventNotifier(obj); err != nil {