MB-8689 Reset index on missed deletions after purge

Change-Id: Ib0996a87979c2f8f6c22c7a4d63738dc9a336cc0
Reviewed-on: http://review.couchbase.org/27863
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>