Ability to trigger snapshot across all peers in a channel via a transaction

As of v2.3 peer admins can request a snapshot be taken at a future block height.

In some scenarios all the organizations on a channel may want to snapshot at the same block height, so that they can compare their snapshot hashes and consider the snapshot an agreed upon 'checkpoint'. This could be accomplished by submitting a transaction that indicates all peers should take a snapshot at that height. Similar to a config transaction, there could be policies governing which organizations must sign off on a snapshot transaction.

Some peers may want to opt-out of the auto-snapshot however, therefore the auto-snapshot behavior should also be governed by a peer config option.