Send actor action probes one at a time

Send actor action probes one at a time

To avoid the scenario whereby the AppSide
delivers a message larger than the maximum
datagram side allowed by the application,
we instead send a single message for each actor.
This prevents us from needing to complicate the
sending logic in Channel.cpp, which would have
been made more complicated should we have wanted
to send variable-sized messages between the
checker and the application side