visitor: add some code to increase data locality

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

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

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