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/test-utils_test.go

@@ -411,8 +411,10 @@ func StartTestPeersRPCServer(t TestErrHandler, instanceType string) TestServer {
 	// Run TestServer.
 	testRPCServer.Server = httptest.NewServer(mux)
 
+	// Set as non-distributed.
+	globalIsDistXL = false
+
 	// initialize remainder of serverCmdConfig
-	srvCfg.isDistXL = false
 	testRPCServer.SrvCmdCfg = srvCfg
 
 	return testRPCServer