Also remember which avatars we didn't find
This massively reduces the amount of network requests, at very little
disk and memory cost (1Mb per 20+k of missing avatars with both backends,
60+k for just Gravatar).
It's even more important when having both backends enabled, in that case
we always did a request for the first backend even if the second one could
provide it from the local cache.
Reviewers: KDE PIM, mlaurent
Reviewed By: KDE PIM, mlaurent
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D7438