Summary:
Add a new API that allows a user to call MultiGet specifying multiple keys belonging to different column families. This is mainly useful for users who want to do a consistent read of keys across column families, with the added performance benefits of batching and returning values using PinnableSlice.

As part of this change, the code in the original multi-column family MultiGet for acquiring the super versions has been refactored into a separate function that can be used by both, the batching and the non-batching versions of MultiGet.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5816

Test Plan:
make check
make asan_check
asan_crash_test

Differential Revision: D18408676

Pulled By: anand1976

fbshipit-source-id: 933e7bec91dd70e7b633be4ff623a1116cc28c8d