Fix configuration handling bugs: (#5473)

* Update the GetConfig admin API to use the latest version of
  configuration, along with fixes to the corresponding RPCs.
* Remove mutex inside the configuration struct, and inside
  notification struct.
* Use global config mutex where needed.
* Add `serverConfig.ConfigDiff()` that provides a more granular diff
  of what is different between two configurations.

cmd/browser-peer-rpc.go

@@ -50,6 +50,10 @@ func (br *browserPeerAPIHandlers) SetAuthPeer(args SetAuthPeerArgs, reply *AuthR
 		return fmt.Errorf("Invalid credential passed")
 	}
 
+	// Acquire lock before updating global configuration.
+	globalServerConfigMu.Lock()
+	defer globalServerConfigMu.Unlock()
+
 	// Update credentials in memory
 	prevCred := globalServerConfig.SetCredential(args.Creds)