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
Polish IterKey and use it in DBImpl::ProcessKeyValueCompaction()

Summary:
1. Polish IterKey a little bit.
2. Turn to use it in local parameter of current_user_key in DBImpl::ProcessKeyValueCompaction(). Our profile showing that DBImpl::ProcessKeyValueCompaction() has about 14% costs in std::string (the base including reading and writing data but excluding compaction filtering), which is higher than it should be. There are two std::string used in DBImpl::ProcessKeyValueCompaction(), compaction_filter_value and current_user_key and it's hard to distinguish the two.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: haobo

CC: igor, yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D17613
Turn on -Wmissing-prototypes

Summary: Compiling for iOS has by default turned on -Wmissing-prototypes, which causes rocksdb to fail compiling. This diff turns on -Wmissing-prototypes in our compile options and cleans up all functions with missing prototypes.

Test Plan: compiles

Reviewers: dhruba, haobo, ljin, sdong

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17649