Leveldb subtree at 1 12 0

Performs the following:

- Brings the LevelDB code base up to 1.12.0
- Incorporates an upstream deadlock fix for Windows condition variables
- Incorporates upstream fix #178: cannot resize a level 0 compaction set
- Limits the rate of compaction to prevent runaway resource usage

Here's the upstream issue regarding the deadlock:
http://code.google.com/p/leveldb/issues/detail?id=149

And the compaction issue:
http://code.google.com/p/leveldb/issues/detail?id=178

This is the commitlog of the subtree:
https://github.com/ripple/leveldb/commits/ripple-fork