consensus: after proposal misses (leader failures) a node should be suspended from proceeding rounds

[sdbondi]

Background

If a node does not propose or proposes invalid blocks, the chain slows down because in order to progress we need to wait for a view change after minimum 12s.

We seek to temporarily suspend offline nodes until they come online to minimise their impact on consensus.

Proposal 1

Suspend:
Each replica tracks proposal misses for each validator member. A proposal miss is detected from a dummy block reaching commit phase(5). In this case a counter is incremented. On propose, a proposer checks for any missed proposal counts that exceed a threshold (MISSED_PROPOSAL_SUSPEND_THRESHOLD) and proposes Command::Suspend(vn_index) in the next block.

Once that block is committed, anytime is_suspended_vn(leader_index(cur_view)) the view is immediately progressed to the next view and the normal leader failure procedure applies (1).

Notes:

1. This could be implemented as the nodes immediately sending a NEWVIEW to the next leader, or
2. the leader(cur_view + 1) could immediately propose a new block with a dummy parent.
3. Validators need to accept the dummy parent block in (2) without NEWVIEW signatures (consensus: prevent unjustified view-change from transmitted dummy blocks (using NEWVIEW signatures) #1160) if the previous leader is suspended.
4. For each of these "forced" dummy blocks, the missed_proposal count is increased (should there be an upper bound?)
5. Updating counts at the commit phase is easiest as we don't have to keep fork-dependent count data

Resume
As votes are received from the suspended node, the missed_proposal count is decremented. Once it reaches 0, a proposer MUST propose Command::Resume(vn_index). Since the counts are only updated on commit blocks, all non-faulty validators have the same counts(1).

Once the resume block has been committed, the validator is reinstated into the round and continues as normal.

Notes:

1. We may want to provide some proof of this in the block, for example, a hash of validator PKs and their corresponding non-zero missed proposal count.

[stringhandler]

Nice, I like it

[sdbondi]

Closed in #1211