[PATCH] bug 3081: comm layer cleanups for TCP acceptor

Attached patch contains the cleanup-comm branch changes to
ListenStateData which make it an AsyncJob (called Comm::TcpAcceptor)
using Subscriptions to generate calls.

This fixes the call re-use problem of bug 3081 and a related FTP data
connection bug.

Alex:
   please test that this does not break with SMP support. I've run it
with 2 workers on a non-SMP machine, but have not checked for anything
more complex.

NP: there are still FTP problems with EPRT/PORT handling of data
connections from servers which do not terminate their directory transfer
with \r\n. That is outside this patch scope.

Amos