split Comm::on_start into two signals (on_send and on_recv)

split Comm::on_start into two signals (on_send and on_recv)

+ the actual start of the Comm occurs when both send and receiver are
  there. This event can be notified by CommImpl::on_start
+ TODO: Comm::on_completion and CommImpl::on_completion are redundant
+ TODO: The CommImpl signals should go back to Comm, but on kernel
  side a CommImpl does not have a piface to notify (yet)