of :meth:`.Session.begin_nested` would fail to correctly
roll back the transaction when a flush error occurred, instead
raising its own exception while leaving the session still
pending a rollback.  [ticket:2718]

message for [ticket:2662]; after_commit() is called within "committed"
state, not prepared, and no SQL can be emitted for prepared or committed
- consolidate state assertions in session transaction, use just one
method
- add more unit tests for these assertions

attempts to emit SQL on a Session within the after_commit()
handler, where there is not a viable transaction in progress.
[ticket:2662]
- rework how SessionTransaction maintains state, using symbols
instead.
- add lots of notes and cross-linking for session events.
- add a link to :func:`.select()` within :meth:`.FromClause.select`.

some of these tests are testing

    a deleted object in the identity map with another
    object of the same primary key would raise a
   "conflicting state" error on rollback(),
    if the replaced primary key were established either
    via non-unitofwork-established INSERT statement
    or by primary key switch of another instance.
    [ticket:2583]

become an externally usable package but still remains within the main sqlalchemy parent package.
in this system, we use kind of an ugly hack to get the noseplugin imported outside of the
"sqlalchemy" package, while still making it available within sqlalchemy for usage by
third party libraries.

    that occurs within Session.commit(), such that the
    extra state added by an after_flush() or
    after_flush_postexec() hook is also flushed in a
    subsequent flush, before the "commit" completes.
    Subsequent calls to flush() will continue until
    the after_flush hooks stop adding new state.
    An "overflow" counter of 100 is also in place,
    in the event of a broken after_flush() hook
    adding new content each time. [ticket:2566]