Convert the http server/mainloop to asyncore

We can avoid most of the Mainloop.Run() code if we use asyncore
for delivering I/O events, and just concentrate on what's missing in
asyncore: singnal handling and timers. This way confd can be ported to
use Mainloop as well.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>