dbwrap_ctdb: only fetch a read-only copy if we had a record already.

dbwrap_ctdb: only fetch a read-only copy if we had a record already.

Because revoking read-only copies of records is expensive, we only
want ctdbd to do it for high-turnover records.  A basic heuristic is
that if we don't find a local copy of the record, don't ask for a
read-only copy.

The fetch itself will cause ctdbd to migrate the record, so eventually
we will have a local copy.  Next time it gets migrated away, we'll
call ctdbd_fetch() with local_copy = true.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>