TransformRep - use array instead of unordered_map

TransformRep - use array instead of unordered_map

Summary:
I'm sending this diff together with https://reviews.facebook.net/D13881 because it didn't allow me to send only the array one.

Here I also replaced unordered_map with just an array of shared_ptrs. This elminated all the locks.

I will run the new benchmark and post the results here.

Test Plan: db_test

Reviewers: dhruba, haobo

Reviewed By: haobo

CC: leveldb
Differential Revision: https://reviews.facebook.net/D13893