[pgp] use full key fingerprints for identity bindings

Fixes #518

Highlights of changes introduced by this MR:

• Introduces versioning in KeyStore database
• Adds migration which resolves all short key IDs to the records corresponding fingerprints
• Drops use of delay_execution because it causes interference between KeyStore instances belonging to different accounts, cancelling writes after migrations. Instead, we dispose need in such ratelimiting with *_nosync private methods used by bulk writes.
• Modifies PGPLegacy.has_valid_key_assigned to make it match learned short fingerprint against key from KeyStore. Key learning is expected to be removed by fix for #519 (closed), so this is just to make everything work in current state.