Summary:
We had two proposals for lock-free commit maps. This patch implements the latter one that was simpler. We can later experiment with both proposals.

In this impl each entry is an std::atomic of uint64_t, which are accessed via memory_order_acquire/release. In x86_64 arch this is compiled to simple reads and writes from memory.
Closes https://github.com/facebook/rocksdb/pull/2861

Differential Revision: D5800724

Pulled By: maysamyabandeh

fbshipit-source-id: 41abae9a4a5df050a8eb696c43de11c2770afdda