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
