Incorrect ViewChange messages consensus calculating

[Toktar]

The simulation test for ViewChange sometimes fails

indy-plenum/plenum/test/consensus/view_change/test_sim_view_change.py

Line 65 in c10090d

def test_new_view_combinations(random):

because of problem with getting a checkpoint on the phase of collecting ViewChange messages in the method calc_checkpoint(). It receives a list of ViewChange messages like a parameter.
If it's a 4 node pool and the list contains the follow ViewChange messages

• ViewChange_1 | checkpoints_ends: 10, 20 | stable_checkpoint: 10
• ViewChange_2 | checkpoints_ends: 0, 10 | stable_checkpoint: 0
• ViewChange_2 | checkpoints_ends: 0 | stable_checkpoint: 0

Then we don't have a strong consensus of 3 (n-f=4-1) checkpoins with the same checkpoint end. It means, that the node can't finish a view change.

Expected problem: Low probability one or more nodes may not finish View Change and after a short period just start a new one.
With an incredibly low probability a pool can freeze with endless view changes. But it can be fixed by POOL_RESTART transaction.

We don’t think we have a big chance to face this case. But we need to remember about it and fix.

[WadeBarnes]

When addressing this issue, please ensure any workarounds such as this are addressed;

indy-plenum/plenum/test/consensus/view_change/test_sim_view_change.py

Lines 90 to 92 in 705582e

	cp = pool.nodes[0]._view_changer._new_view_builder.calc_checkpoint(votes)
	# In some cases checkpoints can't be collected. Uncomment this after fixing issue #1506
	# assert cp is not None