Update metadata version comparison rules in client workflow

[rdimitrov]

The client workflow has a set of version comparisons rules for how to update metadata files. The following PR addresses the differences coming from the fact that when updating not all metadata files should be treated equally.

Fixes #207 and is related to #114

Notes:

• root should be updated only when there's a newer version
• timestamp should be updated only when there's a newer version AND the referenced snapshot version is not less than the one in the already trusted timestamp
• snapshot should be updated only when its version is the same as the one referenced in the trusted timestamp AND the version of the referenced targets metadata (incl. delegated targets files) is equal or newer than the ones in the trusted snapshot.
• targets should be updated only when its version is the same as the one referenced in the trusted snapshot
• In regards to #114, for timestamp, the decision is to take into account the new timestamp metadata file only if its version is not less than or equal to the trusted one, so this PR addresses one of the use cases mentioned in the issue.

[joshuagl]

@mnm678 if you're still comfortable with the change could you re-approve? Thank you.