source3/lib/util_tdb.c: operation timeout support for TDB2.

source3/lib/util_tdb.c: operation timeout support for TDB2.

TDB2 doesn't have (the racy) signal pointer; the new method is to
override the locking callbacks and do the timeout internally.

The technique here is to invalidate the struct flock when the timeout
occurs, so it works even if it happens before we enter the fcntl() call.

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