thorium: clear message tag padding when setting cache entry

Otherwise, the random bits in the padding will interfere with
the message cache hit rate.

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_actor: reinstate assertion in thorium_actor_name()

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

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_message: metadata size is known at compilation time

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: improve information about cache tags

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add function thorium_cache_tag_signature()

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
message_cache: add code to track bug # TRACK_REGRESSION_2014_12_30_BUG

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