Support `READ COMMITTED`

Currently, Dolt only supports the transaction isolation level `REPEATABLE READ`. This is acceptable for most use cases, but is not fully MySQL compatible. In particular, Django database migrations use the `READ COMMITTED` isolation level during database initialization.

[MySQL docs](https://dev.mysql.com/doc/refman/8.0/en/innodb-transaction-isolation-levels.html#isolevel_read-committed)

A Django-specific migration [workaround](https://github.com/dolthub/go-mysql-server/pull/528) has been included in the sql engine behind a feature flag, but this is not a general purpose solution for `READ COMMITTED`.