consensus: accept Dusk-signed Emergency Block

[fed-franz]

Partially implements #1234

New features:
It enables the acceptance of an Emergency Block (EB) with the following properties:
- iteration = u8::MAX
- timestamp > prev_block.timestamp + MIN_EMERGENCY_BLOCK_TIME, where MIN_EMERGENCY_BLOCK_TIME is of MAX_STEP_TIMEOUT x 3 (MAX_ITER_TIMEOUT) times CONSENSUS_MAX_ITER + 1. This value aims at ensure enough time is given to provisioners to produce a block before an Emergency Block can be accepted.
- signed by Dusk (DUSK_CONSENSUS_KEY)

No Attestation is needed to accept an EB.

For what concerns finality, the Emergency Block is consider as being produced at iteration CONSENSUS_MAX_ITER (i.e., the one after the last consensus iteration)

The EB is repropagated upon acceptance (unlike other Block messages)

---

Refactors:

• on_block_event: reduces indentation and improves code clarity by splitting attach_att_if_needed into is_block_attested and attach_blk_att
• execute_checks: change disable_att_check into check_attestation
• rusk/node: remove duplicate Dusk key loading
• generate_block: move seed computation and "block generation elapsed-time log" from generate_candidate_message to generate_block; take voters from ru
• remove iteration check from message deserialization (this also allows to have a standalone definition of CONSENSUS_MAX_ITER, instead of tieing it to MESSAGE_MAX_ITER

[fed-franz]

This PR has been successfully tested:

• locally, with locally crafted emergency blocks
• on testnet, with no apparent regressions

[herr-seppia]

LGTM