Rename '$wgStreamLoggers' => '$wgRCEngines'

Rename '$wgStreamLoggers' => '$wgRCEngines'

The name '$wgStreamLoggers' captures the direction in which I'd like to see
this evolve, but for the time being it is a misnomer, giving the false
impression that it is more general than it in fact is. I did not bother
deprecating the old name since it was only merged to master yesterday.

Also renamed: RecentChange::getStreamEngine -> RecentChange::getEngine (for
consistency with the change to the config var name).

Change-Id: I0d7b08e591e897b24528ea0981a6fcb93cfe8831