Reduce implicit Comm::Connection closures

Reduce implicit Comm::Connection closures

* permits the ConnOpener parent Jobs to abort the opening by canceling
  their callbacks early.
* makes FTP, forwarding and ConnStateData components cancel pending comm
  opens from their destructors (swanSong in ConnStateData).

This seems to reduce the cycles consumed by ConnOpener and also the Async
call delays somewhat in the cases where client connections are aborted.