Validator messages pruning logic

[Ivshti]

It should work like this:

• drop Heartbeats that are older than a threshold
• drop Accounting messages that are older than a threshold; can always be replayed via eventAggrs
• drop NewState/ApproveState older than a threshold, except ones with the stateRoot of the lastApproved (to ensure we can't drop our last approved pair)

see below

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after a few discussions with @samparsky, it would be sufficient to:

• drop Heartbeats that are older than threshold
• drop all validator messages for expired channels (channel.validUntil); this can be done via an additional prune script
• add a maximum channel validity that's accepted (ethereum adapter); should be 1 year

(re accounting messages: removing them is extremely dangerous, since if we remove the last available one, then everything is practically broken cause accounting will start from 0)