Increase priority of DKG-related consensus transactions on protocol level

[AnnaShaleva]

Rationale

Consensus nodes have to send some DKG-related transactions for proper dBFT epoch setup. Currently there's no way prioritize these transactions on the protocol level. See more details on the problem in #332 (comment).

Implementation

Implement protocol-level prioritization of consensus transactions, something similar to N3's HightPriority attribute. The caveat is not to change the existing Ethereum protocol significantly, so it's a matter of discussion.

[roman-khimov]

Depends on #46.

[vncoelho]

agreed