Add pooled_connection option

[baarden]

feat: option for useful cert refresh behavior for pooled connections

When a pool of connections is being maintained by SQLAlchemy, there is no need for the connections to do their own maintenance, since SQLAlchemy already closes unhealthy connections and opens new connections as needed. And because pooled connections do not get closed actively, the current refresh behavior can result in too many connections being held open, and surges of refreshes every 55 minutes if multiple pools were opened at the same time.

Similarly, the current behavior of immediately retrying a failed refresh can lead to a thundering herd of refreshes if the Cloud SQL API requests-per-minute quota has been exceeded. In a managed pool it will be safer to wait 60 seconds for the next quota period.

Separately, the connector is throwing asyncio errors (below), which are corrected by adding an explicit close() call to the __client_session after use. This action is moved to a separate method to avoid flake8 complexity complaints.

ERROR:asyncio:Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x10f1fd850>
ERROR:asyncio:Unclosed connector

Changes:

• Adds a pooled_connection option to Connector and Instance which modifies the behavior of certification refreshes. When true, timed refreshes are disabled. Also, failed refreshes are retried after 60 seconds.
• A new _close_session method is added to Instance to prevent unclosed ClientSession errors.
• Tests also require a GOOGLE_APPLICATION_CREDENTIALS envvar value, so that suggestion is added to CONTRIBUTING.md.

Testing: Passing tests continue to pass with these changes.

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[jackwotherspoon]

Closing due to inactivity on #415