tokenrenewer: stop trying to renew token when kite went down
tokenrenewer: stop trying to renew token when kite went down

This PR handles a situation, when kite missed
disconnect signal (observed to happen with XHR transport).

Due to this, kite floods the log files with:

	ERROR    token renewer: no kites found (057542be08a10e72) Cannot renew token for Kite: 091de227-f943-4aa5-98f0-024a709a4f57 I will retry in 10 seconds...

koding/kite,rjeczalik/kite,koding/kite,rjeczalik/kite,rjeczalik/kite,koding/kite