tdb: delete from CCAN. This was an early experiment in putting code into CCAN, but TDB is a public library distributed as part of the Samba sources; there is no reason to publish it here (especially now the unit tests are also in Samba).
tdb: add Bob Jenkins lookup3 hash as helper hash. This is a better hash than the default: shipping it with tdb makes it easy for callers to use it as the hash by passing it to tdb_open_ex().
tdb: fix tdb_check() on read-only TDBs to actually work. But make sure we can still do tdb_check() inside a transaction (weird, but we previously allowed it).