[SC-2880] Batch P2P Messages for a single multisig stage

[AlastairHolmes]

Currently the multisig outputs the messages it would like to send via p2p on a channel where each message is sent as a separate pair of account id and message. I would like to refactor this so that all the messages to be sent in a single stage as grouped into a single struct/item sent on the channel. This would allow us to remove the duplicate of the sent message in the broadcast case. And make the multisig tests significantly simpler, as waiting for all the messages from a completed stage, is a single wait (and the number of waits isn't dependent on the number of participants). Also it would help avoid repeatedly encoding the message before sending it.

TODO More detail.

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@msgmaxim

[AlastairHolmes]

#966

[AlastairHolmes]

Going to wait for @msgmaxim 's unit test PR before implementing this.