message_cache: add function to print message cache statistics

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
unitig_visitor: print message cache statistics

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
message_cache: store the expected reply action for caching

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add actor name when printing message cache information

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
message_cache: improve robustness by checking the reply action

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>