Data file column tombstone cleanup

[westonpace]

With the addition of #2639 it is now possible to introduce tombstoned columns with merge_insert. A user reported that this led to a surprising amount of disk usage since the files with tombstones are still "active" (and won't be cleaned up) but the tombstoned data is redundant.

We could examine what percentage of a file is tombstoned and, if this is large (configurable threshold), we can rewrite the file and remove the tombstoned columns.

This can be run as part of cleanup or potentially compaction / optimize.