Feature: to optimise the current mining logic

[zzzckck]

Rationale

To follow the FF design more strictly, blocks of "no-turn-with-attestation" should be preferred over "in-turn-without-attestation".
It could avoid validator delay broadcast too much.

Implementation

For a validator, during mining, it imported a block:

• if the imported block is an in-turn-with-attestation: stop minning
• if the imported block is an in-turn-without-attestation: keep mining and collect votes, once enough vote to get the attestation, just broadcast the block to flip the imported one.

Notice: should be more cautious to check if it will bring more reorg or not first.

[buddh0]

yes, it may cause more reorg.
let's pending this issue, it may can be applied after FF improved.