The new locateUsersWatchingTitle implementation could end up returning
thousands of users, currently on enwiki there are 25 titles with more
than 10k subscribed users and aprox 550 titles with more than 1k subscribed
users.
This switches the user collection to an iterator based implementation so that
we no longer need to have the entire users list at any one time.
Change-Id: I3d3fa9328f348bb48682d3658622952ce82d3925
