cbus: code review for dynamic cbus routing

* move cbus declaration to cbus.c, since going forward
  it's a private singleton
* rename cbus_item to cbus_endpoint. It represents the state
  of a single cord in the cbus (but in the future, a single cord
  may have more than one endpoint.
* move declaration of cbus_endpoint to cbus.c
* factor out cbus_find_endpoint()
* add/update comments
* move cbus_init()/cbus_free() to main.cc
* replace pthread_cond_signal() with pthread_cond_broadcast(),
  since in theory there can be many waiters on an endpoint
  to join.