Randomness DKG would stall epoch switch if one shard is completely offline

[SkidanovAlex]

Currently the aggregate phase would stall if at least one shard is completely offline.

The solution should be to detect it happened (e.g. by observing that the aggregation phase didn't make progress for some number of steps), and to remove all the shares committed in the stalled shard from the aggregation (and to restart the aggregation).

[stale]

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[bowenwang1996]

Randomness beacon near/NEPs#206