Pass top level compute arguments through to compute inside `find_objects`

Suggestion from Marvin, about how we might improve the `find_objects` function https://github.com/dask/dask-image/pull/240#discussion_r681072020
> Hmm I'm wondering whether arguments to the top level compute call, such as the chosen scheduler, would need to be forwarded to this call. Maybe there's a way to access these here?

Follow up to https://github.com/dask/dask-image/pull/240

Read the `find_objects` entry point and the linked pull request discussion first; trace where the top-level compute call invokes the nested compute. Done means the relevant compute arguments, including the scheduler choice, reach the inner call and the behavior is covered by a regression test.

Written by the indexing model from the issue text.