Convert ganeti-masterd's main thread to mainloop

Not much changes with this patch. The main loop for the IOServer is
repaced by mainloop.Run() and the main thread now uses asyncore to
handle connections to the master socket. Once it accepts them, though,
it just pushes them to the current infrastructure, and everything
proceeds as before.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>