remove _legacy_descriptors, provide mechanism for clearing _instance_key

[jek]

the _instance_key, _sa_session_id and _entity_name compatibility descriptors should be axed completely, per irc discussion.

but we need some mechanism (function, session method?) for manipulating or at least clearing _instance_key / InstanceState.key and allowing users to kick a persistent or detached instance all the way down to transient.

[Mike Bayer]

the first part is done, the second part is not critical for 0.5.0. I'd propose:

   from sqlalchemy.orm import make_transient
   make_transient(instance)

[Mike Bayer]

491f6796f869d597922018660cc491441a878543

[Mike Bayer]

Removing milestone: 0.6.0 (automated comment)