stats: optionally also show how many entries are deleted entries

[wojas]

It would be very helpful if the stats command can also output how many entries per DBI are marked as deleted.

Since this requires a full scan of the LMDB, this should be disabled by default and require a flag like --full.

As a workaround, you can find the number of deleted entries with something like:

lightningstream snapshots dump ... | grep flags=01 | wc -l