Summary:
Add a new table property, rocksdb.num.range-deletions, which tracks the
number of range deletions in a block-based table. Range deletions are no
longer counted in rocksdb.num.entries; as discovered in PR #3778, there
are various code paths that implicitly assume that rocksdb.num.entries
counts only true keys, not range deletions.

/cc ajkr nvanbenschoten
Closes https://github.com/facebook/rocksdb/pull/4016

Differential Revision: D8527575

Pulled By: ajkr

fbshipit-source-id: 92e7edbe78fda53756a558013c9fb496e7764fd7