Fix AlwaysAcceptingBroker to send exactly one contract offer to each known peer.

Change-Id: Ie2b8a6c10d4a633418e6e22ac1fb4cf606cbbd36
Reviewed-on: http://naan.mimuw.edu.pl:8001/67
Tested-by: Nebulostore Nebulostore <nebulostore@gmail.com>
Reviewed-by: Jadwiga Kanska <jadzz.k@gmail.com>