Use `autoflush=False` for SQLAlchemy sessions

[r4victor]

By default, SQLAlchemy sessions work in autoflush=True mode. In this mode, all pending changes to the session are committed on any session.execute, so even selects may trigger write operations. It can be a big problem on SQLite leading to long write transactions and "database is locked" errors.

Switch to autoflush=False. This is a huge change to ORM semantics but luckily for us, the dstack code does not rely on autoflush much. There are few places that need to be fixed:

• _get_next_instance_num()

Potentially:

• generate_*_name functions like this but only if used to create multiple objects in a session.

On a related note, ensure there are no long write transactions.