Add changelog

Save retrieved keys to the db

lint

Fix and de-brittle remote result dict processing

Use query_user_devices instead, assume only master, self_signing key types

Make changelog more useful

Remove very specific exception handling

Wrap get_verify_key_from_cross_signing_key in a try/except

Note that _get_e2e_cross_signing_verify_key can raise a SynapseError

lint

Add comment explaining why this is useful

Only fetch master and self_signing key types

Fix log statements, docstrings

Remove extraneous items from remote query try/except

lint

Factor key retrieval out into a separate function

Send device updates, modeled after SigningKeyEduUpdater._handle_signing_key_updates

Update method docstring