- rework ColumnCollection to no longer persist "all_col_set"; we don't

- rework ColumnCollection to no longer persist "all_col_set"; we don't
need this collection except in the extend/update uses where we
create it ad-hoc.  simplifies pickling.  Compatibility with 1.0
should be OK as ColumnColleciton uses __getstate__ in any case
and the __setstate__ contract hasn't changed.
- Fixed bug in :class:`.Table` metadata construct which appeared
around the 0.9 series where adding columns to a :class:`.Table`
that was unpickled would fail to correctly establish the
:class:`.Column` within the 'c' collection, leading to issues in
areas such as ORM configuration.   This could impact use cases such
as ``extend_existing`` and others.  fixes #3632