Move all verification into setters on repo

[diogomonica]

All the verifications should be moved down to the setters on the repository. The only thing we have to do at the store level is the size verification.

[diogomonica]

Depends on #563

[cyli]

As per #555 (comment), either the setters on the repo or other functions will perform per-metadata specific content verification. After which we probably will no longer need signed.Verify, which verifies both metadata and signatures, and just signed.VerifySignatures.

[cyli]

Note: on further IRL discussion, we are going to do a walk+visitor pattern to load a TUF repo, and verification should happen on a verification visitor.

[riyazdf]

this is related to the builder, and we will have verification in the walker. Moving to 0.3