Track latest sequences on receive, and set depends_on on send

[mkysel]

The depends_on field is used in #1553 to sort messages upon receive.

This issue tracks the client that sends the message to the payer.

Depends_on is part of AAD and is encrypted by the client, but readable by the other parties in the system.

The payer/node might reject a bogus dependency.

The field can either be:

• the latest sequenceID of originator 1 (the blockchain)
• or the latest sequenceID of originator 1 for that group

Option 2) might lead to less delays and sorting issues.