Support Merge with wide-column entities in iterator (#10941)

Support Merge with wide-column entities in iterator (#10941)

Summary:
The patch adds `Merge` support for wide-column entities in `DBIter`. As before, the `Merge` operation is applied to the default column of the entity; any other columns are unchanged. As a small cleanup, the PR also changes the signature of `DBIter::Merge` to simply return a boolean instead of the `Merge` operation's `Status` since the actual `Status` is already stored in a member variable.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10941

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D41195471

Pulled By: ltamasi

fbshipit-source-id: 362cf555897296e252c3de5ddfbd569ef34f85ef