upnp: always delete released mappings

Currently, a released mapping is not deleted but rather set as
available for future use. However, in some circumstances, the
socket bound to the mapped port may take some time to be completed
closed and ready for reuse. Which means if the mapping is reused
shortly after it was released, the socket binding may fail  on the
mapped port.
To prevent this race, a relased  mapping is always deleted.
Note that in the current implementation, new mapping will be
automatically requested if needed.

Gitlab: #413

Change-Id: I9e6dce912aa14e29050669940b2846038b0a591c