add shield() in aexit

add shield() in aexit

Added ``asyncio.shield()`` to the connection and session release process
specifically within the ``__aexit__()`` context manager exit, when using
:class:`.AsyncConnection` or :class:`.AsyncSession` as a context manager
that releases the object when the context manager is complete. This appears
to help with task cancellation when using alternate concurrency libraries
such as ``anyio``, ``uvloop`` that otherwise don't provide an async context
for the connection pool to release the connection properly during task
cancellation.

Fixes: #8145
Change-Id: I0b1ea9c3a22a18619341cbb8591225fcd339042c