Rename Nonce to BatchID

[evan-gray]

As an integrator, nonce is an overloaded term which is not very self-descriptive. It is easy to misinterpret that this might be like a wallet nonce where the caller should be tracking and incrementing a counter, but this is essentially what the sequence number is. With #1679 it is proposed that this field be used to enable and group messages into batches.

Should this field be renamed to something more obvious like groupId or batchId? Are there any concerns about renaming? As far as I understand the string is not used anywhere and the data would remain unchanged - this should merely be a cosmetic change for clarity’s sake.