WriteAtPrepare: Efficient read from snapshot list

WriteAtPrepare: Efficient read from snapshot list

Summary:
Divide the old snapshots to two lists: a few that fit into a cached array and the rest in a vector, which is expected to be empty in normal cases. The former is to optimize concurrent reads from snapshots without requiring locks. It is done by an array of std::atomic, from which std::memory_order_acquire reads are compiled to simple read instructions in most of the x86_64 architectures.
Closes https://github.com/facebook/rocksdb/pull/2758

Differential Revision: D5660504

Pulled By: maysamyabandeh

fbshipit-source-id: 524fcf9a8e7f90a92324536456912a99aaa6740c