use custom pool for everything in systemp db

[maxdml]

This PR:

• Ensure we use the user pgxpool for every interaction with the system DB, if provided
• Moves the notification listener to using a connection from from said pool, rather than using a standalone connection
• Make the notification listener loop resilient to connection failures (addresses Resilient notification listener #94)

Note that OnNotification has to be set before a connection is started (which is too late once Acquired from the pool), which is why the logic to handle notifications is now inlined in the listener.