Transaction ID should be returned in a single header

[eddyashton]

Currently we return 2 headers (view and seqno) - these should be combined into a single header. Other APIs which take these fields should be updated to take a single string.

[achamayou]

@lynshi your input on naming constraints, if any, would be most welcome.

[eddyashton]

We discussed this briefly last week - I think we're all happy with x-ms-ccf-transaction-id, so that's what I'll aim for initially.

We don't plan to make this configurable, since there isn't a need for it right now (and it introduces a subtle error class, and complicates receipts, and would need to be synced across client SDKs including the CCF Python client).

[achamayou]

Sounds good, let's go with that.