kv/RocksDBStore: Add CompactOnDeletion support

kv/RocksDBStore: Add CompactOnDeletion support

This commit adds support to compact column families when a certain number
of tombstone entries have been observed within a certain sliding window
during iteration.  It only helps when itereating over entries already in
SST files and not when iterating over ranges in memtables.

Likely we will still need to provide a mechanism to flush memtables and
compact column families once a certain number of rmkey or rm_range_key
calls are made.

Signed-off-by: Mark Nelson <mnelson@redhat.com>