thorium: add adaptive actor API call

While a distributed system provides more distributed memory than
a single node (1024 Blue Gene/Q nodes provide 16 TiB of DDR3 memory),
there can be some issues with scaling actor computations that send
small messages (with a low amount of associated work).

Fangfang and I discussed how we can scale actor
applications that send a lot of small messages.

The concept of adaptive actor theoretically imposes a lower bound
on the multiplexer message promotion rate (the promotion rate is
real_message_count / original_message_count).

By getting as close as possible to this lower bound, the running time
can be decreased.

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
Reviewed-by: Fangfang Xia <fangfang@anl.gov>
genomics: unitig_manager is now an adaptive actor

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
Reviewed-by: Fangfang Xia <fangfang@anl.gov>