Modify StoreKeyFetcher to read from server_keys_json. (#15417)

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.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/15417.bugfix b/changelog.d/15417.bugfix
new file mode 100644
index 0000000000..300635cbdc
--- /dev/null
+++ b/changelog.d/15417.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where cached key results which were directly fetched would not be properly re-used.