Summary:
Allow the users to specify the target index partition size.

With this patch an index partition is cut before its estimated in-memory size goes above the configured value for metadata_block_size. The filter partitions are still cut right after an index partition is cut.
Closes https://github.com/facebook/rocksdb/pull/2041

Differential Revision: D4780216

Pulled By: maysamyabandeh

fbshipit-source-id: 95a0831