Ensure compatibility with bluesky-widgets databroker search widget

[danielballan]

The bluesky-widgets databroker search widget is currently known to be incompatible with the databroker 2.x prereleases. There are two ways to address this:

1. Short-term fix: One issue that Databroker no longer supports arbitrary RawMongo queries. We have a RawMongo shim that makes some effort to support a restricted subset of queries and reduce the amount of backward-incompatibility with existing code. But the shim is not quite sophisticated enough to cover the use case needed by bluesky-widgets. With more effort, it could be.
2. Long-term fix: Update bluesky-widgets to use Databroker 2.x fully, making use of new features like sorting and proper pagination. Update search widgets for Databroker 2.x bluesky-widgets#181