This is important for the web UI where a user might want to clear out
multiple GPG keys in a row. If we were to do this by firing off
multiple requests, we would introduce write contention on the gpg-keys
ref, likely causing lock failures on the update since we don't
(currently) spin and retry.
Rather than adding a new endpoint, reuse the existing POST endpoint
for this purpose. The response from this endpoint is now a map similar
to listing all keys, except it only lists keys that were modified by
the operation. Deleted keys have no fields in the response.
Change-Id: Id4b7ec651380720249f7623e9824e8b116962f7d
eab3aff03a