doc: Add CODE DESIGN

src/query/CODE_DESIGN.md

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+The rules by which the query service code works:
+
+- Unlike table-service, a grpc connection is created per endpoint only once, and is reused for all sessions with that endpoint
+
+- When a new session is created, a grpc stream is created with the attach() command
+
+- The attach-stream remains open for the entire lifetime of the session, including when it
+  returns to the pool of free sessions.
+
+- If attach-stream breaks, the session is considered not reusable.  But its current use, if any, is not interrupted.
+
+- A session can be terminated if the exec method returns an error that the session is broken - same logic as in table-service
+
+- There is no re-attach mechanism at the moment, but it is possible that it will appear in the future
+
+- In the first approximation of the service implementation, only the sequential mode of receiving result sets is used
+
+- It still may be worth to make an experimental mixed mode, since we will have to compare performance anyway
+
+- Maximize the use of session pool management logic and error handling logic from table-service
+
+- Data output is impleted via async iterator. Important: Make sure that in all error scenarios, all
+  Promises that have gotten out and have not yet completed will return reject