MemTableIterator not to reference Memtable

Summary: In one of the perf, I shows 10%-25% CPU costs of MemTableIterator.Seek(), when doing dynamic prefix seek, are spent on checking whether we need to do bloom filter check or finding out the prefix extractor. Seems that  more level of pointer checking makes CPU cache miss more likely. This patch makes things slightly simpler by copying pointer of bloom of prefix extractor into the iterator.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: ljin

CC: igor, dhruba, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D17247