tokenrenewer: improve fix for a panic #187
tokenrenewer: improve fix for a panic #187

The error returned by kontrol is different than
the ErrKitNotAvailable, that's why the assert
on message content.

Moreover the renewLoop lifetime handling was
improved:

- "kite not found" errors do not cause the goroutine
  to exit - they're just ignored instead, in case
  a heartbeat went through and kite was again
  registered to kontrol without local Kite knowing it
  (no OnConnect handler gets executed then); this
  is an assumption, I did not manage to write a test
  to prove it

- we wait for previous renewLoop to die before
  starting a new one

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