http: Unlisten sockets after all workers quit
http: Unlisten sockets after all workers quit

This (almost) move only ensures the event base loop doesn't exit before
HTTP worker threads exit. This way events registered by HTTP workers are
processed and not discarded.

peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin