message_cache: fix format for message cache statistics

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
cache_tag: add function to get destination from tag

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
message_cache: verify the validity of the reply message for caching

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