Allow transaction listeners/plugins to get the current org.geotools.data.Transaction

Using the approach proposed here:

http://sourceforge.net/mailarchive/forum.php?thread_name=CAEwWEk1jTrFc%3DWKjDfN052KugZLLQrTN4Y-kHC9PQxArB6rQiw%40mail.gmail.com&forum_name=geoserver-devel

Tested with my own plugin and it appears to work.