Allow a unique ID to be passed with FTS requests

When submitting FTS queries, it would be useful if a unique ID could be submitted with the payload. The ID would then be used within an server logging and also returned in response payload.

This would allow correlation between client and server logs, to help diagnose problems. This is in relation to Per-Operation Tracing, where other services provide a similar feature. For example, N1QL provides a client_context_id field.