Add server support code to HubServer

This adds the code necessary to make HubServer manage the listening
sockets.  Provision is made for the server to be started (separately
from initialization), stopped, and shut down.
Resolve hub addresses

Tendril wants addresses, not hostnames, so update parse_hub() to
perform a name resolution step.
Select correct endpoint for manager

In Tendril, outgoing connections must be made from a manager of the
same address family.  This adds IPv6 support for HeyU by ensuring
that the submitter obtains the correct manager based on the address
family that the hub resolves to.