PlainTableBuilder::Add() to use local char array instead of reused std::string as tmp buffer

Summary: Our profile shows that in one of the applications, 5% of the CPU costs of PlainTableBuilder::Add() are spent on std::string stacks. By this simple change, we avoid this global reusable string. Also, we avoid another call of file appending, which probably gives another 2%.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: haobo

CC: igor, yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D17601
Fix ASAN issue
speed up db_bench filluniquerandom mode

Summary:
filluniquerandom is painfully slow due to the naive bitmap check to find
out if a key has been seen before. Majority of time is spent on searching
the last few keys. Split a giant BitSet to smaller ones so that we can
quickly check if a BitSet is full and thus can skip quickly.

It used to take over one hour to filluniquerandom for 100M keys, now it
takes about 10 mins.

Test Plan:
unit test
also verified correctness in db_bench and make sure all keys are
generated

Reviewers: igor, haobo, yhchiang

Reviewed By: igor

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D17607
Improved CompressedCache

Summary:
This is testing behavior that was reported in https://github.com/facebook/rocksdb/issues/111

No issue was found, but it still good to commit this and make CompressedCache more robust.

Test Plan: this is a plan

Reviewers: ljin, dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17625