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/browser-peer-rpc.go

@@ -78,8 +78,11 @@ func (br *browserPeerAPIHandlers) SetAuthPeer(args SetAuthPeerArgs, reply *Gener
 
 // Sends SetAuthPeer RPCs to all peers in the Minio cluster
 func updateCredsOnPeers(creds credential) map[string]error {
-	// Get list of peers (from globalS3Peers)
-	peers := globalS3Peers.GetPeers()
+	// Get list of peer addresses (from globalS3Peers)
+	peers := []string{}
+	for _, p := range globalS3Peers {
+		peers = append(peers, p.addr)
+	}
 
 	// Array of errors for each peer
 	errs := make([]error, len(peers))