WatchedItemStore: Update process cache when adding items

WatchedItemStore: Update process cache when adding items

This way, when submitting POST to action=watch, the response will
not show "Unwatched" in the navigation 'content action' tabs.

This was previously the case on any wiki installation that has
more than 1 database as the skin would call User::isWatched(),
which would query stale data from a slave.

This doesn't yet account for the reverse use case (removing from
watchlist). That's to be addressed in a later commit.

Bug: T28292
Change-Id: Ie4b69c985815a77b70692db0c4dbf52e1a6a018d