Summary:
I've added functions to the C API to support WriteBatchWithIndex as requested in #1833.

I've also added unit tests to c_test

I've implemented the WriteBatchWithIndex variation of every function available for regular WriteBatch.  And added additional functions unique to WriteBatchWithIndex.

For now, the following is omitted:
  1. The ability to create WriteBatchWithIndex's custom batch-only iterator as I'm not sure what its purpose is.  It should be possible to add later if anyone wants it.
  2. The ability to create the batch with a fallback comparator, since it appears to be unnecessary.  I believe the column family comparator will be used for this, meaning those using a custom comparator can just use the column family variations.
Closes https://github.com/facebook/rocksdb/pull/1985

Differential Revision: D4760039

Pulled By: siying

fbshipit-source-id: 393227e