sql: detection of single-statement implicit transactions broken in batch

[ajwerner]

Describe the problem

In #76834 we changed the behavior of how a batch of statements are
executed when sent in a single protocol message. A number of features in cockroach use the TxnImplicit field of the EvalContext to decide whether the statement being executed is a single-statement explicit transaction. Below is a non-exhaustive list:

• ALTER COLUMN TYPE
• ALTER TABLE DROP COLUMN
• CREATE TABLE AS SELECT
• REFRESH MATERIALIZED VIEW
• enabling the declarative schema changer
• deserializing the session state
• SET CLUSTER SETTING
• SET LOCAL
• DECLARE CURSOR
  • Maybe we also don't want to allow this in the single batch, I don't know
• crdb_internal.gc_tenant
• Some code about statement stats and their transaction fingerprint.
• Metrics tracking in here.

This might be as simple as moving everything to use the AutoCommit concept instead of the Implicit concept. I'm not sure. I think AutoCommit might permit these things to happen as the last statement in a multi-statement "implicit" transaction. That worries me.

Jira issue: CRDB-14467

[ajwerner]

The work in #78512 attempted to address some of this, but is incomplete

[rafiss]

further work on #78512 seems to have completely addressed this