cache_tag: add function to print a cache tag for debugging

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_message_cache: save request message as a cache tag

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
message_cache: make sure memory pool is not set to a random pointer

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