vtctldclient ExecuteFetchAsDBA (and similarly the vtctl and vtctlclient commands) now reject multi-statement SQL with error.

For example, vtctldclient ExecuteFetchAsDBA my-tablet "stop replica; change replication source to auto_position=1; start replica will return an error, without attempting to execute any of these queries.

Previously, ExecuteFetchAsDBA silently accepted multi statement SQL. It would (attempt to) execute all of them, but:

• It would only indicate error for the first statement. Errors on 2nd, 3rd, ... statements were silently ignored.
• It would not consume the result sets of the 2nd, 3rd, ... statements. It would then return the used connection to the pool in a dirty state. Any further query that happens to take that connection out of the pool could get unexpected results.
• As another side effect, multi-statement schema changes would cause schema to be reloaded with only the first change, leaving the cached schema inconsistent with the underlying database.

ExecuteFetchAsDBA does allow a specific use case of multi-statement SQL, which is where all statements are in the form of CREATE TABLE or CREATE VIEW. This is to support a common pattern of schema initialization.