client: Verify and fix the version comparison done for metadata files

[rdimitrov]

Description:

• Verify the usage of “less than vs less than or equal to” done for version comparison throughout the code
• The confusion comes that the spec uses the same text in multiple scenarios that are not the same
• Meta versions can be equal, but metadata versions probably should not be. For everything other than timestamp, it does not matter. For timestamp, spec always expects to download the latest and use that.
• For example, it’s normal for the timestamp version to change but still points to the same snapshot version.
• Note that fixing this implies a change in the specification too.

[mnm678]

Can you cross-post an issue to the specification repo? It'd be great to fix this there first so that we stay compliant with the spec.

[rdimitrov]

Linked to #282

[rdimitrov]

Closing this one in favour of #282