deprecate order_by parameter on mapper

[Mike Bayer]

This is a little used parameter with very little impact on current code, however as per #3223 there are edge cases that are not worth supporting. Deprecation here would likely be indefinite but new or currently unworking use cases would remain unsupported.

[Mike Bayer]

Deprecate Mapper.order_by

This is an old parameter no longer relevant to how SQLAlchemy works, once the Query object was introduced. By deprecating it we establish that we aren't supporting non-working use cases and that we encourage applications to move off of the use of this parameter.

Fixes: #3394 Change-Id: I25b9a38142a1537bbcb27d3e8b66a8b265140072

→ <<cset 23825a259176>>