tdb: add Bob Jenkins lookup3 hash as helper hash.

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().

This version taken from CCAN and modified, which took it from
http://www.burtleburtle.net/bob/c/lookup3.c.

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