[PREVIEW] Comm::Connection async jobs

To get around some layering violations between client and server side
I've found it easier to continue the rollout of Comm::Connection through
client-side that to work up a working hack.

The attached patch contains only the src/comm/ API pieces for review so
we can ensure they are done properly before much more testing and work
is done to make use of them.

Comm::Connection - data object. stores the FD, local IP+port, remote
IP+port, and comm flags relevant to the socket connection.

Comm::ConnAcceptor - was ListenStateData, but now converted to AsyncJob.
Takes a connection to listen on and emits active Comm::Connection
objects for new client connections.

Comm::ConnConnector - take a closed template Comm::Connection and make
it an active connection.

If these are right I'll follow up with to core code changes done so far
to use them.