Follow-up from "WIP: extend source api and remove private use from workspace plugin"

The following discussion from !1651 (merged) should be addressed:

• @juergbi started a discussion:

  I think we can remove this. However, it looks like it requires a fix in Stream.shell() as that's missing a check for _source_cached(), so it may make sense to handle that in a separate MR.

• remove call to commit() in Source()._generate_key

• fix missing cache check in shell