Introduce a fast unlink function for rmr feature

[anysql]

What would you like to be added:

Just check the source code, the rmr will call "Unlink" function for file deletion, it will update the parent directory attributes (nlink/mtime etc) and statistics (quota limits) for each file. Since we are going to drop the directory, there is no need for these extra steps, at least we can consider a batch pipeline mode for less network roundtrips.

Why is this needed:

Improve the rmr performance, may be 2x or more faster.

[davies]

We should update the quota, and skip some of operations on parent.