Check block requests response in higher level code instead

[tomaka]

Right now, the SyncService::blocks_query function downloads the block from a peer then verifies whether the body matches the header.

This has two issues:

• In order for this to work, one need to also download the header, which is often necessary. There are several TODOs in the code where we download the header when unnecessary.
• Inconsistency. None of the other networking requests have this verification. Block requests are the only requests that have this, because it's actually possible to perform some verification, whereas for other requests it's not.

In #1200 I'm planning to remove this check.
This PR is a preliminary change to make the high-level code verify the block headers and bodies, instead of the low-level code.

Note that the syncing code already verifies whether headers in responses match the expected hash. As for bodies, I've added a TODO in the optimistic syncing. Since this doesn't concern the light client it isn't critical.

A side change is that I realize that chain_getBlock directly returns the justifications downloaded from the network without verifying them. I've removed this and mentioned it in the CHANGELOG.