[1.8.x] Fixed #24921 -- set_autocommit(False) + ORM queries.

This commits lifts the restriction that the outermost atomic block must
be declared with savepoint=False. This restriction was overly cautious.

The logic that makes it safe not to create savepoints for inner blocks
also applies to the outermost block when autocommit is disabled and a
transaction is already active.

This makes it possible to use the ORM after set_autocommit(False).
Previously it didn't work because ORM write operations are protected
with atomic(savepoint=False).

Backport of 91e9f1c9 from master