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/lockinfo-handlers.go

@@ -99,7 +99,7 @@ func getSystemLockState() (SystemLockState, error) {
 func (c *controlAPIHandlers) remoteLockInfoCall(args *GenericArgs, replies []SystemLockState) error {
 	var wg sync.WaitGroup
 	var errs = make([]error, len(c.RemoteControls))
-	// Send remote call to all neighboring peers to restart minio servers.
+	// Send remote call to all neighboring peers fetch control lock info.
 	for index, clnt := range c.RemoteControls {
 		wg.Add(1)
 		go func(index int, client *AuthRPCClient) {
@@ -133,7 +133,7 @@ func (c *controlAPIHandlers) RemoteLockInfo(args *GenericArgs, reply *SystemLock
 	return nil
 }
 
-// LockInfo - RPC control handler for `minio control lock`. Returns the info of the locks held in the cluster.
+// LockInfo - RPC control handler for `minio control lock list`. Returns the info of the locks held in the cluster.
 func (c *controlAPIHandlers) LockInfo(args *GenericArgs, reply *map[string]SystemLockState) error {
 	if !isRPCTokenValid(args.Token) {
 		return errInvalidToken
@@ -167,3 +167,20 @@ func (c *controlAPIHandlers) LockInfo(args *GenericArgs, reply *map[string]Syste
 	// Success.
 	return nil
 }
+
+// LockClearArgs - arguments for LockClear handler
+type LockClearArgs struct {
+	GenericArgs
+	Bucket string
+	Object string
+}
+
+// LockClear - RPC control handler for `minio control lock clear`.
+func (c *controlAPIHandlers) LockClear(args *LockClearArgs, reply *GenericReply) error {
+	if !isRPCTokenValid(args.Token) {
+		return errInvalidToken
+	}
+	nsMutex.ForceUnlock(args.Bucket, args.Object)
+	*reply = GenericReply{}
+	return nil
+}