thorium: use the multiplexer only on specific messages

Multiplexing every message with a buffer size below the threshold
slows down things because such messages will only leave the node
when the timeout is reached (or in the next call if the timeout is 0).

This patch instead requires that actions to be multiplexed
be explicitly registered in the multiplexer policy.

Link: https://github.com/GeneAssembly/biosal/issues/749
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: reduce the size of message metadata

The routing algorithm is not implemented, so we don't need
the routing attributes in messages.

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: refine the message deleguation model

Before, the worker pool would push a message in the mailbox
of an actor and then notify the worker of that event.

Now, the worker pool simply push the message to the worker,
and the worker is responsible for giving the message to the actor
and also scheduling the actor.

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: push directly in mailbox if the destination is the same actor

Link: https://github.com/GeneAssembly/biosal/issues/750
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
scheduler: use better status names

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add code to send to local actors in a worker

Link: https://github.com/GeneAssembly/biosal/issues/748
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>