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.
