try to gracefully close even in terminate

try to gracefully close even in terminate

Adjusted the asyncpg dialect such that when the ``terminate()`` method is
used to discard an invalidated connection, the dialect will first attempt
to gracefully close the conneciton using ``.close()`` with a timeout, if
the operation is proceeding within an async event loop context only. This
allows the asyncpg driver to attend to finalizing a ``TimeoutError``
including being able to close a long-running query server side, which
otherwise can keep running after the program has exited.

Fixes: #10717
Change-Id: Iaba0aeb67873a7a2b3981d43f4eb663005057309