socketsrc: Add `connection-closed-by-peer` signal

socketsrc: Add `connection-closed-by-peer` signal

This provides notification that the socket in use was closed by the peer
and gives an opportunity to replace it with a new one which is not
closed, allowing reading from many sockets in order.

I use this in pulsevideo to implement reconnection logic to handle the
pulsevideo service dieing, such that is can be restarted without
disrupting downstream.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=739546