Block::Iter::PrefixSeek() to have an extra check to filter out some false matches

Summary:
In block based table's hash index checking, when looking for a key that doesn't exist, there is a high chance that a false block is returned because of hash bucket conflicts. In this revision, another check is done to filter out some of those cases: comparing previous key of the block boundary to see whether the target block is what we are looking for.

In a favored test setting (bloom filter disabled, 8 L0 files), I saw about 80% improvements. In a non-favored test setting (bloom filter enabled, files are all in L1, files are all cached), I see the performance penalty is less than 3%.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: ljin

Subscribers: wuj, leveldb, zagfox, yhchiang

Differential Revision: https://reviews.facebook.net/D20595