[MEDIUM] stream-interface: add a ->release callback

[MEDIUM] stream-interface: add a ->release callback

When a connection is closed on a stream interface, some iohandlers
will need to be informed in order to release some resources. This
normally happens upon a shutr+shutw. It is the equivalent of the
fd_delete() call which is done for real sockets, except that this
time we release internal resources.

It can also be used with real sockets because it does not cost
anything else and might one day be useful.