Verify blocks before storing them as pending

[MegaRedHand]

We currently store pending blocks in our DB, which makes our P2P layer serve them to peers on request. We should instead first do as many checks we can (or at least verify their signature), before storing them.

[Aliemeka]

Will love to take a look into this @MegaRedHand

[MegaRedHand]

Sure, but I suggest coming up with a list of checks to do before opening a PR