Tracking: introduce ExecuteMultiFetchAsDba gRPC method

[shlomi-noach]

Feature Description

As a reminder, we recently fixed an ExecuteFetchAsDBA issue where:

• ExecuteFetchAsDBA would allow multiple queries in a single call.
• The error result was only checked for the first query, ignored for 2nd, 3rd, ... queries.
• Query results for 2nd, 3rd, ... queries were left unconsumed, and returned with the connection back into the pool, thereby leaking results into other users of the pool.

The fix included:

• Lower lever error in ExecuteFetch, and
• Higher level check for multiple results in rpc_query.go.

However, the work is incomplete. We had to exclude a specific use case in rpc_query.go: ApplySchema can execute potentially multiple DDLs and runs these via ExecuteFetchAsDBA.

The roadmap to the complete fix is:

v20:

• Introduce a ExecuteMultiFetchAsDba gRPC call, which safely runs multiple statements, checks error and breaks on first error, and returns multiple result sets.
• Introduce a vtctldclient ExecuteMultiFetchAsDBA command, which calls ExecuteMultiFetchAsDba gRPC method and returns a list of result sets.

v21:

(skipped/delayed to next version)

v22:

• ApplySchema()/TabletExecutor to use ExecuteMultiFetchAsDba instead of ExecuteFetchAsDba.

v23:

• ExecuteFetchAsDBA in rpc_query.go to disallow any form of multiple queries.

Use Case(s)