Query missing cross-signing keys on local sig upload

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Save retrieved keys to the db

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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

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Only fetch master and self_signing key types

Fix log statements, docstrings

Remove extraneous items from remote query try/except

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Factor key retrieval out into a separate function

Send device updates, modeled after SigningKeyEduUpdater._handle_signing_key_updates

Update method docstring