Use fingerprints as key index in DB4 backend

Up until now the DB4 backend has used the 64 bit key ID as the index
into the key database. The fingerprint has always been a better idea
and it's easy to create a v3 key that causes a collision, so switch to
using the fingerprint in the DB4 backend.

These changes to not require an upgrade to an existing DB4 setup; keys
will get transitioned to the new storage method as they are updated in
the keyserver.