Align on Transport GID and Liveliness GUID

[Yadunund]

At present, entities like pub/subs are assigned an rmw_gid_t gid value and this is attached to the payload. However, GraphCache relies on a std::size guid to unique identify entities. The size_t is a std::hash of the liveliness token which is now unique.

It would be best if we could combine the two somehow.

1. Either derive the 16byte GID from the liveliness token or
2. Append the 16byte GID to the liveliness token.

Personally I'd prefer 1) to since the liveliness tokens are already several bytes.