Add a subkey ID mapping DB for DB4 backend

With the increased use of signature subkeys and the fact long
keyids are being used to do HKP lookups we need to store a mapping
of the subkey long keyids to primary key ids. Previously the fact
we did this for the 32 bit subkey id was enough. Add a new DB for
the DB4 backend that can map the 64 bit subkey ID to the 64 bit
primary key ID.