Rethink when to call BlueskyEventStream._load_partitions

[gwbischof]

I think this may be an issue if the if load_partitions is first called before all of the documents are available. The next time you call read() on the stream, you may not get all of the documents, because load_partitions is not called a second time.

https://github.com/bluesky/databroker/blob/master/databroker/core.py#L1397

[danielballan]

Indeed, DataSource does not have the reload() concept that Catalog does. Perhaps it should.