tdb: cleanup: rename GLOBAL_LOCK to OPEN_LOCK.

The word global is overloaded in tdb.  The GLOBAL_LOCK offset is used at
open time to serialize initialization (and by the transaction code to block
open).

Rename it to OPEN_LOCK.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from commit 7ab422d6fbd4f8be02838089a41f872d538ee7a7)