renames _collection to _db in User
adds handler for CheckingActivity in User - prevents the User from getting
cleaned up if they are logged in or have games in progress.

this means that logged in users will stay in memory for the life of the
app.
adds _loadFromDb to User.replace so the new user takes on the latest
data from the old user even if it hasn't been saved to the db yet.

(it hasn't been decided yet whether stuff will be saved to the db
immediately, so this may be unnecessary)