Replace observe on all users with one observe per connection.
glasser review comments
Remove redundant call to 'removeTokenFromConnection'
Poll for the observe to appear in a test.
Clean up observes created for connections that were closed before the
observe was set up.
Remove `connectionsClosedBeforeObserve`.

Instead of inserting into `connectionsClosedBeforeObserve`, we can just
delete the null placeholder from `userObservesForConnections`, and then
check for a deleted placeholder before storing the observe once it's
been set up.
Merge branch 'remove-users-observe' into devel