Before this change:
* `PerspectivesKeyFetcher` and `ServerKeyFetcher` write to `server_keys_json`.
* `PerspectivesKeyFetcher` also writes to `server_signature_keys`.
* `StoreKeyFetcher` reads from `server_signature_keys`.
After this change:
* `PerspectivesKeyFetcher` and `ServerKeyFetcher` write to `server_keys_json`.
* `PerspectivesKeyFetcher` also writes to `server_signature_keys`.
* `StoreKeyFetcher` reads from `server_keys_json`.
This results in `StoreKeyFetcher` now using the results from `ServerKeyFetcher`
in addition to those from `PerspectivesKeyFetcher`, i.e. keys which are directly
fetched from a server will now be pulled from the database instead of refetched.
An additional minor change is included to avoid creating a `PerspectivesKeyFetcher`
(and checking it) if no `trusted_key_servers` are configured.
The overall impact of this should be better usage of cached results:
* If a server has no trusted key servers configured then it should reduce how often keys
are fetched.
* if a server's trusted key server does not have a requested server's keys cached then it
should reduce how often keys are directly fetched.
