tdb: cleanup: always grab allrecord lock to infinity.

We were previously inconsistent with our "global" lock: the
transaction code grabbed it from FREELIST_TOP to end of file, and the
rest of the code grabbed it from FREELIST_TOP to end of the hash
chains.  Change it to always grab to end of file for simplicity.

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