stasis: Reduce calculation of stasis message type hash.

When the stasis cache is used a hash is calculated for
retrieving or inserting messages. This change calculates
a hash when the message type is initialized that is then
used each time needed. This ensures that the hash is
calculated only once for the message type.

Change-Id: I4fe6bfdafb55bf5c322dd313fbd8c32cce73ef37