[patch] Add after_begin hook to SessionExtension

[Former user]

I've been working on tidying up the zope transaction integration story. This patch adds an after_begin hook to SessionExtension called when a connection is added to a SessionTransaction. This enables the session to 'join' the zope transaction on first use.

[Mike Bayer]

this has been implemented (don't have the changeset # handy, it was some weeks ago).