* lisp/gnus/gnus-gravatar.el (gnus-gravatar-insert): Fix check for
repeated gravatars, which is now easier to trigger now that things
are more asynchronous.
* lisp/image/gravatar.el (gravatar--service-libravatar): Fetch the
data asynchronously (bug#40676).
(gravatar-service-alist): Adjust all providers so they are
asynchronous.
(gravatar-build-url): Adjust caller to be asynchronous.
(gravatar-retrieve): Ditto.
(gravatar-retrieve-synchronously): Ditto.