Avoid multiple writes to changetags table in recentchanges_save hook

Avoid multiple writes to changetags table in recentchanges_save hook

Currently, extensions that add tags within the recentchanges_save
hook directly call ChangeTags::addTags. This often results in
consecutive writes to the changetags table.
This provides a addTags method to RecentChange so extensions
can simply use it when they want a tag added. And all the actual
tagging is done once at the end.

Change-Id: I8df2fd983c12632337e8d2922fa357808482338c