Change UserAddressManager to do polling instead of a pushing

[hackaugusto]

Abstract

The current implementation of the UserAddressManager is callback based, changes to a user's presence will trigger the user_presence_changed_callback, and changes to the address' reachability will trigger address_reachability_changed_callback. This model has caused a few problems:

• Crashes because the underlying map could be iterate over by one thread and modified by another.
• Bugs because the callbacks was not called on all code paths.
• A more complex/confusing interface because of the additional callbacks and a similarity to a thread interface.

Motivation

To prevent problems like the ones mentioned above, the code should be simplified to do polling instead of pushing.

[konradkonrad]

Is this still the case? cc @fredo @ulope