Added clear subcommand for control lock (#3013)

Added clear subcommand for control lock with following options:

```
  3. Clear lock named 'bucket/object' (exact match).
    $ minio control lock clear http://localhost:9000/bucket/object

  4. Clear all locks with names that start with 'bucket/prefix' (wildcard match).
    $ minio control lock --recursive clear http://localhost:9000/bucket/prefix

  5. Clear all locks older than 10minutes.
    $ minio control lock --older-than=10m clear http://localhost:9000/

  6. Clear all locks with names that start with 'bucket/a' and that are older than 1hour.
    $ minio control lock --recursive --older-than=1h clear http://localhost:9000/bucket/a
```

cmd/control-handlers.go

@@ -261,7 +261,7 @@ func (c *controlAPIHandlers) ServiceHandler(args *ServiceArgs, reply *ServiceRep
 	return nil
 }
 
-// LockInfo - RPC control handler for `minio control lock`. Returns the info of the locks held in the system.
+// TryInitHandler - generic RPC control handler
 func (c *controlAPIHandlers) TryInitHandler(args *GenericArgs, reply *GenericReply) error {
 	if !isRPCTokenValid(args.Token) {
 		return errInvalidToken