Separate grouping and sorting semantics

[stress-tess]

This issue contains the requested functionality previously captured in Issue #846 that was not addressed by PR #1330

• Separate grouping and sorting semantics
  • argsort and coargsort should actually sort the array(s). Currently, calling coargsort on a list including a Strings or Categorical will only group, not sort.
  • GroupBy should guarantee grouping, but not necessarily sorting. Strings and Categorical should have separate APIs for sorting and grouping, and GroupBy should call the latter.

The high-level actions required by this issue are:

• [x] Create a server message specifically for GroupBy that takes over the grouping logic currently employed by coargsort
• [ ] Modify coargsort to actually sort the data, rather than just grouping it (for strings)

@reuster986 or anyone else, feel free to add anything I missed from the original issue

[stress-tess]

Note this issue is related to and summarizes the conversation in #677. I would recommend closing that issue in favor of this one