tdb2: use tdb->flags & TDB_RDONLY instead of tdb->read_only for TDB1 code.

tdb2: use tdb->flags & TDB_RDONLY instead of tdb->read_only for TDB1 code.

There's also a semantic change here: for tdb1, being read-only meant no
locking, and it was an error to try to lock a r/o database.  For TDB2, you'd
need to specify TDB_NOLOCK, which suppresses locking silently.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from CCAN commit 4dc29a338fadeac805b369b4b0851c02f1b152c7)