Use four 32-bit signed parts to represent the client id

[esteve]

This PR replaces unsigned 64-bit integers with signed 32-bit integers to ensure portability.

Connects to ros2/rmw_connext#31

[mikaelarguedas]

@tfoote or @ros2/team can you confirm that this change has been considered unnecessary because the RTPS specification states that the GUID has to fit on 16 octets ? (12 for the prefix and 4 for the entity) Relevant sections of the spec: 8.2.1.2 and 8.4.2.1

Or because we assume the machines can emulate 64 bits by using 2 32-bit registers internally without requiring the application to explicitly split them into 32-bit variables?

[tfoote]

I think this was closed during a meeting when I was logged in. But I believe that this was closed with the expectation that 64bit integers would work.

The logic was related to connext's query parser: ros2/rmw_opensplice#88 (comment) but I think we determined that we didn't need to use that? I'm not sure though.

[mikaelarguedas]

sounds good thanks! deleting this branch then. If anybody has more insights feel free to comment here