- lazy loads for relationship attributes now use

- lazy loads for relationship attributes now use
the current state, not the "committed" state,
of foreign and primary key attributes
when issuing SQL, if a flush is not in process.
Previously, only the database-committed state would
be used.  In particular, this would cause a many-to-one
get()-on-lazyload operation to fail, as autoflush
is not triggered on these loads when the attributes are
determined and the "committed" state may not be
available.  [ticket:1910]

- A new flag on relationship(), load_on_pending, allows
the lazy loader to fire off on pending objects without a
flush taking place, as well as a transient object that's
been manually "attached" to the session. Note that this
flag blocks attribute events from taking place when an
object is loaded, so backrefs aren't available until
after a flush. The flag is only intended for very
specific use cases.