Detect a format change in the payloader and post an element message.
Catch the format change element message in the client-source and use it
to update the format and possible formats in the source and source-outputs.
Add a global object that can be used to track outstanding fds for a
client. This can later also be used to limit the number of fds per
client.
Use this in the payloader to track fds. This makes it possible to track
the fds even when the client reconnects.
Set the client patch on the socket so that we can use it to track the
fds we sent to this client.
When the client disappears, release all outstanding fds.
Rework stream connect/disconnect from start/stop
Start the stream in setcaps when the format is known.
When the stream is already started in setcaps, send a format_change
message instead.
