Show contents of all submitted transactions in relay logs

[svyatonik]

We have now Sent transaction to {} node: {hash} message in logs for all submitted transactions. But messages about transaction contents are always different, so it isn't always clear about what this transaction is. Especially if we are submitting multiple transaction to different nodes at once.

Ideally we need to have something like that: Sent transaction to {} node: {hash}. Type: MessageDeliveryTransaction(FromMillau). So not the whole tx contents, just its "digest" instead.

[svyatonik]

With standalone relayers (#3002), this issue won't cause that much pain - logs are much more clear and we always know what are those transcations.