Introduce HRegion#doMiniBatchMutation()

From Anoop under thread 'Can there be a doMiniBatchDelete in HRegion':

The HTable#delete(List<Delete>) groups the Deletes for the same RS and make one n/w call only. But within the RS, there will be N number of delete calls on the region one by one. This will include N number of HLog write and sync. If this also can be grouped can we get better performance for the multi row delete.

I have made the new miniBatchDelete () and made the HTable#delete(List<Delete>) to call this new batch delete.
Just tested initially with the one node cluster. In that itself I am getting a performance boost which is very much promising.
Only one CF and qualifier.
10K total rows delete with a batch of 100 deletes. Only deletes happening on the table from one thread.
With the new way the net time taken is reduced by more than 1/10
Will test in a 4 node cluster also. I think it will worth doing this change.