dbwrap_ctdb: handle read-only records.

dbwrap_ctdb: handle read-only records.

The new read-only record flags make determining if we can use a record
a bit more complex, so extract it into its own function.

The OLD logic was:
1) If the record doesn't exist, we can't use it.
2) If we are the dmaster for the record, we can use it.

The new logic is:
1) If the record doesn't exist, we can't use it.
2) If we are the dmaster for the record, we can use it IF we only
   want read-only access, OR there are no read-only delegations.
3) If we are not dmaster, we can only use it if we want read-only
   access and it is marked as a read-only copy.

This logic is unused until the next patches which begin to ask
for read-only copies of records.

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