Allow pbm-agent to run with no database connection

Currently, pbm-agent refuses to start if it cannot connect to a healthy local mongod. On a sharded cluster, also the config server replica set needs to be reachable and healthy.

The pbm-agent should still be able to start (log a warning message saying no DB connection was possible). This way we can for example kick off a physical restore even if the database is not healthy. Obviously logical restore won’t be possible in this mode, but we can print an error message if the user tries to do it.