[1.23wmf15] Make SiteStats (re)initializing more sane

Don't set ss_active_users back to its default in SiteStatsInit::refresh.
Also remove SiteStatsInit::update and make refresh() protected.
Also don't consider ss_active_users in SiteStats::isSane as that value
isn't going to be fixed by the following SiteStatsInit::refresh call.

Change-Id: I268599be96106e1175fdf9750a2adc9468ebc93c
(cherry picked from commit d8c03733718ac9eedabad7e4fa569426a56acc31)