A problem in connection closing.

* A send connect to B (lightid=1024)
  A.next_lightid = 1025
  A.outgoing = 1

* B send connect to A (lightid=1024)
  B.next_lightid = 1025
  B.outgoing = 1

* A send close connect to B
  A.outgoing = 0
  send CloseSocket to B (last_incoming=0)
  enter Closing state

* B send close connect to A
  B.outgoing = 0
  send CloseSocket to A (last_incoming=0)
  enter Closing state

* B got connect from A
  recover from Closing state.
  incomings.add(1024)

* B got close connect from A
  incomings.remove(1024)

* A got connect from B
  recover from Closing state.
  incomings.add(1024)

* A got close connect from B
  incomings.remove(1024)

In the end, both A and B's outgong==0 and incomings empty, but the heavyweight connection is not closed.
Maybe we should try CloseSocket again when we got close connection request, and find that outgoing==0 and incomings is empty?

Start by tracing the connection-closing state handling for the A/B sequence described in the issue, especially the transitions involving Closing, outgoing, and incomings. Reproduce the simultaneous connect-and-close scenario and verify that the heavyweight connection is closed once both outgoing is zero and incomings is empty.