Summary:
In CompactRange, if there is no key in memtable falling in the specified range, then flush is skipped.
This PR extends this skipping logic to SST file levels: it starts compaction from the highest level (starting from L0) that has files with key falling in the specified range, instead of always starts from L0.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6482

Test Plan:
A new test ManualCompactionTest::SkipLevel is added.

Also updated a test related to statistics of index block cache hit in db_test2, the index cache hit is increased by 1 in this PR because when checking overlap for the key range in L0, OverlapWithLevelIterator will do a seek in the table cache iterator, which will read from the cached index.

Also updated db_compaction_test and db_test to use correct range for full compaction.

Differential Revision: D20251149

Pulled By: cheng-chang

fbshipit-source-id: f822157cf4796972bd5035d9d7178d8dfb7af08b