Verify block certificate prior to broadcast

[goshawk-3]

Describe the bug
On receiving a block from Kadcast network, we used to fully verify it before re-propagating it to the next bucket. Now to speed up block broadcast to all provisioners, we re-propagate it without any checks.

Checking a certificate and block header should be a precondition for a block propagation.

[herr-seppia]

In this case, txs root should be reintroduced

[goshawk-3]

In this case, txs root should be reintroduced

This should be addressed with a separate issue that may also take into account the backward compatibility.