ReplicatedPG: replace clean_up_local with a debug check

Stray objects should have been cleaned up in the merge_log
transactions.  Only on the primary have those operations
necessarily been flushed at activate().

Fixes: 5084
Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: start flush on primary only after we process the master log

Once we start serving reads, stray objects must have already
been removed.  Therefore, we have to flush all operations
up to the transaction writing out the authoritative log.

Signed-off-by: Samuel Just <sam.just@inktank.com>