[3.1.1 backport] cbgt importlistener leaves open a connection to a bucket after database is deleted

In investigating CBG-2905:

Inside cbgt code, there's a gocbcore agent running for each bucket that runs. These are cached on a bucket level, so it doesn't open a new connection if you close a DatabaseContext and re-open it. However, if you delete a bucket, it will try to connect repeatedly to the non existent bucket and log a warning 1 sec, forever.

I'm not sure how to shut down this agent. In order to isolate this warning to cbgt, you need CBG-2982. https://github.com/torcolvin/CBG-2905 contains the reproduction recipe for this.