MB-8780: don't purge deletions that are not yet indexed

Change-Id: I4854626a4bf7a6e0d5f429999a72701c09b050cf
Reviewed-on: http://review.couchbase.org/28036
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>