lock: Moving locking to handler layer. (#3381)

This is implemented so that the issues like in the
following flow don't affect the behavior of operation.

```
GetObjectInfo()
.... --> Time window for mutation (no lock held)
.... --> Time window for mutation (no lock held)
GetObject()
```

This happens when two simultaneous uploads are made
to the same object the object has returned wrong
info to the client.

Another classic example is "CopyObject" API itself
which reads from a source object and copies to
destination object.

Fixes #3370
Fixes #2912

cmd/xl-v1-object.go

@@ -62,11 +62,6 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64, length i
 		return traceError(errUnexpected)
 	}
 
-	// Lock the object before reading.
-	objectLock := nsMutex.NewNSLock(bucket, object)
-	objectLock.RLock()
-	defer objectLock.RUnlock()
-
 	// Read metadata associated with the object from all disks.
 	metaArr, errs := readAllXLMetadata(xl.storageDisks, bucket, object)
 	// Do we have read quorum?
@@ -222,10 +217,6 @@ func (xl xlObjects) GetObjectInfo(bucket, object string) (ObjectInfo, error) {
 		return ObjectInfo{}, err
 	}
 
-	objectLock := nsMutex.NewNSLock(bucket, object)
-	objectLock.RLock()
-	defer objectLock.RUnlock()
-
 	info, err := xl.getObjectInfo(bucket, object)
 	if err != nil {
 		return ObjectInfo{}, toObjectErr(err, bucket, object)
@@ -485,11 +476,6 @@ func (xl xlObjects) PutObject(bucket string, object string, size int64, data io.
 		}
 	}
 
-	// Lock the object.
-	objectLock := nsMutex.NewNSLock(bucket, object)
-	objectLock.Lock()
-	defer objectLock.Unlock()
-
 	// Check if an object is present as one of the parent dir.
 	// -- FIXME. (needs a new kind of lock).
 	if xl.parentDirIsObject(bucket, path.Dir(object)) {
@@ -606,10 +592,6 @@ func (xl xlObjects) DeleteObject(bucket, object string) (err error) {
 		return err
 	}
 
-	objectLock := nsMutex.NewNSLock(bucket, object)
-	objectLock.Lock()
-	defer objectLock.Unlock()
-
 	// Validate object exists.
 	if !xl.isObject(bucket, object) {
 		return traceError(ObjectNotFound{bucket, object})