Allow HA/Http events to interrupt InterfaceMgr main-thread select

HA lease updates can get backlogged because underlying IOService(s) in HTTP gets starved by the maing-thread select() in InterfaceMgr. This select blocks waiting for DHCP traffic or other registered IO for upto 1 second at time. This means that during relatively slow or intermittent DHCP traffic, this select() can block for large amounts of time and HA/Http ready handlers pile up.

By registering ExternalSockets with InterfaceMgr for Http connections, and possibly adding WatchSocket(s) to the HA layer to trigger on write events and/or timeouts, we should be able to get much better behavior.