Allow submitting multiple transaction at once

[abailly-iohk]

Why

In the current version of the protocol, each transaction is submitted to the Head and included in a snapshot and therefore approved and signed individually. This can be problematic when a use case requires a sequence of transactions to happen atomically, something which popped up recently in a discussion with @Trymrt.

What

• Allow a party to submit multiple transactions at once
• Ensure those transactions are processed and included (or not) in a snapshot as a single block by the Head network

How

The changes to make seem relatively simple:

• Make NewTx accept a list of transactions
• Make ReqTx contain a list of transactions