Summary:
Partition Index blocks and use a Partition-index as a 2nd level index.
The two-level index can be used by setting
BlockBasedTableOptions::kTwoLevelIndexSearch as the index type and
configuring BlockBasedTableOptions::index_per_partition
t15539501
Closes https://github.com/facebook/rocksdb/pull/1814
Differential Revision: D4473535
Pulled By: maysamyabandeh
fbshipit-source-id: bffb87e