[JENKINS-16342] Improving responsiveness of asynchPeople when Gravatar plugin is in use.

This change does not necessarily improve total performance, since the avatar is still computed.
But (1) the computation is correctly done in the work thread, not in the HTTP response thread;
(2) the computation is done just once for a given User, which could reduce load when many AJAX checks are done.(cherry picked from commit c757e654)

Conflicts:
	changelog.html