use QueuePool for sqlite file databases

use QueuePool for sqlite file databases

The SQLite dialect now defaults to :class:`_pool.QueuePool` when a file
based database is used. This is set along with setting the
``check_same_thread`` parameter to ``False``. It has been observed that the
previous approach of defaulting to :class:`_pool.NullPool`, which does not
hold onto database connections after they are released, did in fact have a
measurable negative performance impact. As always, the pool class is always
customizable via the :paramref:`_sa.create_engine.poolclass` parameter.

Fixes: #7490
Change-Id: I5f6c259def0ef43d401c6163dc99f651e519148d