Rethink transaction sync endpoint

[Mirko-von-Leipzig]

As per #1593 (comment), we have some issues with our SyncTransactions endpoint.

• TransactionHeader gRPC definition does not map to the protocol definition
• Users cannot form TransactionId from our gRPC response (original issue Add transaction_id field to TransactionRecord #1272)
• NoteSyncRecord is no longer bounded due to the recent addition of note attachments
  • We could instead send just a note metadata "header", users can request the attachments via the notes endpoint

We need to design a cohesive (and sensible) schema change here which either aligns the gRPC definitions with the protocols, or name things differently so that its not confusing. The former seems ideal, but this may result in excess data that isn't technically required - or just be painful to get.

Supersedes #1272

[SantiagoPittella]

I split this issue in 2 subissues:

1. TxHeader enabling TxId computation
2. NoteMetadataHeader to avoid unbounded size notes.

[bobbinth]

@Mirko-von-Leipzig @SantiagoPittella - this has already been addressed, right?

[SantiagoPittella]

Yes 👌🏼