Extend database backends to support fetching by key fingerprint

Up until now the database backends have supported fetching a specific
key only by 64 bit key id. We should really have been using the
fingerprint where possible. Add a new backend function, fetch_key_fp,
to allow us to fetch using the fingerprint, and a generic implementation
that just truncates the fingerprint to get the 64 bit key id (valid for
v4 only). The HKP, keyd and dynamic backends gain full passthrough
support for retrieval by fingerprint with this commit.