get rid of autocommit behavior

Postgres has no concept of autocommit. Any statement that takes place outside of an explicit transaction is automatically committed. Effectively: autocommit is always on in Postgres.

This means it must be impossible to set `autocommit = off` in doltgres.

Start by locating DoltgreSQL's handling of the autocommit setting and transaction boundaries. Confirm how PostgreSQL treats statements outside explicit transactions, then identify the existing tests for setting autocommit. Done means attempts to set autocommit off are rejected or otherwise impossible, with tests covering that behavior.

Written by the indexing model from the issue text.