upnp: Connectivity change handling and clean-up

This patch introduces three major changes:
1- When a "Connectivity Change" notification is received from the
configuration manager, the current IGDs and the provisioned mappings
were reset and a new search and provisioning was started anew.
Now, the reset/restart is performed only if the host address actually
changes.
2- Many code simplification and clean-up. Obsolete code used to
handle asynchronous operations on libupnp was also removed.
Most of the calls to libupnp are now synchronous. Blocking calls
are dispatched on the internal execution queue (worker thread).
3- Add synchronization to avoid thread race at shutdown

Gitlab: #416

Change-Id: I26970d78db494795f23379c7a9af5a20a665bb06