Includes:
- https://github.com/bitcoin-core/leveldb/pull/2: Prefer std::atomic over MemoryBarrier (Pieter Wuille)
- https://github.com/bitcoin-core/leveldb/pull/5: Move helper functions out of sse4.2 object (Cory Fields)
- https://github.com/bitcoin-core/leveldb/pull/6: Fixes typo (Dimitris Tsapakidis)
- https://github.com/bitcoin-core/leveldb/pull/10: Clean up compile-time warnings (gcc 7.1) (Matt Corallo)
- https://github.com/bitcoin-core/leveldb/pull/11: fixup define checks. Cleans up some oopses from #5 (Cory Fields)