simplify ThreadLocalPtr a little bit

Summary:
make singleton a static member instead of dynamic object. This should
also avoid the race on unique_ptr

Test Plan: make all check

Reviewers: igor, haobo, sdong

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18177
get rid of shared_ptr in memtable.cc

Summary: Get rid of the devil. Probably won't impact anything on the perf side.

Test Plan: make all check

Reviewers: igor, haobo, sdong, yhchiang

Reviewed By: haobo

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D18153