ice transport: use dedicated struct to hold pending upnp state

* fix: use the same mutex in callback and to wait
* fix: notify with mutex locked
* fix: avoid accessing this from callback, preventing potential use-after-free
* allows to stop waiting for upnp mappings immediately
 when the transport is stopped
* perform single allocation of shared state,
 and free the memory of mutex and cv after mapping completes

Change-Id: I573937d2e8a90b31501a237607bbc7f52942bcea