Avoid per-key linear scan over snapshots in compaction (#4495)

Avoid per-key linear scan over snapshots in compaction (#4495)

Summary:
`CompactionIterator::snapshots_` is ordered by ascending seqnum, just like `DBImpl`'s linked list of snapshots from which it was copied. This PR exploits this ordering to make `findEarliestVisibleSnapshot` do binary search rather than linear scan. This can make flush/compaction significantly faster when many snapshots exist since that function is called on every single key.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4495

Differential Revision: D10386470

Pulled By: ajkr

fbshipit-source-id: 29734991631227b6b7b677e156ac567690118a8b