instrumentation: instrumentation for locks. (#2584)

- Instrumentation for locks.
- Detailed test coverage.
- Adding RPC control handler to fetch lock instrumentation.
- RPC control handlers suite tests with a test RPC server.

cmd/control-shutdown-main.go

@@ -39,7 +39,7 @@ var shutdownCmd = cli.Command{
 USAGE:
   minio control {{.Name}} http://localhost:9000/
 
-EAMPLES:
+EXAMPLES:
   1. Shutdown the server:
     $ minio control shutdown http://localhost:9000/