Clarify client workflow when no new metadata is available

The client workflow describes in detail how to update metadata in order to download a target. However, it should be clarified how use local trusted metadata in order to download a target even if no new metadata is available from remote.

Details per role

• For root there actually are instructions to just proceed with target downloading if no new root version is available from remote, according to the version in the unsigned filename, and to the version in the signed metadata. The latter means that filename and signed version are inconsistent, which seems like a repo error. Should that really be ignored? (cc @rdimitrov, #209)

• For timestamp instructions were also added in #209. However, the new wording -- "In case they are equal, discard the new timestamp metadata and abort the update cycle. This is normal and it shouldn't raise any error." -- seems contradictory. Should this rather be -- "In case they are equal, discard the new timestamp metadata and go to 5.5"? (re-cc @rdimitrov)

• For snapshot and targets the instructions are to always download the version listed in timestamp (for snapshot) or snapshot (for targets) respectively. Would it be a fair optimization to only download, if the listed versions for either of them isn't already available locally?

Start by reading the client workflow in tuf-spec.md, especially the root and timestamp passages linked in the issue and the snapshot and targets instructions. Compare the local-metadata cases across all four roles and resolve the inconsistencies and optimization question raised here and in #209. Done means the specification clearly states when to reuse, discard, or download metadata and what happens next.