Summary:
There is a small `SingleDelete` related optimization in the
`CompactionIterator` code: when a `SingleDelete`-`Put` pair is preserved
solely for the purposes of transaction conflict checking, the value
itself gets cleared. (This is referred to as "optimization 3" in the
`CompactionIterator` code.) Though the rest of the code got updated to
support `SingleDelete`'ing blob indexes, this chunk was apparently
missed, resulting in an assertion failure (or `ROCKS_LOG_FATAL` in release
builds) when triggered. Note: in addition to clearing the value, we also
need to update the type of the KV to regular value when dealing with
blob indexes here.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7904

Test Plan: `make check`

Reviewed By: ajkr

Differential Revision: D26118009

Pulled By: ltamasi

fbshipit-source-id: 6bf78043d20265e2b15c2e1ab8865025040c42ae