thorium: call rand_r() in actor instead of in worker

Link: https://github.com/GeneAssembly/biosal/issues/792
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add a function to get the worker name of an actor

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: assign an actor to a worker when it is spawned

Otherwse, placement depends on the order of arrival of
messages.

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
performance: limit the number of targets to reduce memory usage

On Cray XE6 with 256 nodes, the number of actors is:
irb(main):001:0> 256*22*100
=> 563200

This patch selects 22 target actors per node which are allowed
to receive ACTION_PING messages. All actors still send
ACTION_PING messages however.

Link: https://github.com/GeneAssembly/biosal/issues/785
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>