2.0.x yd4 backport fix for 632 stop plugins in reverse order

This is a backport of 6f0578aeee34145df0c51cd1d93630b501626865 to Play 2.0.x.

This fixes the errors on shutdown (including in tests) when any asynchronous use of DB connections triggers errors due the BoneCP connection pool being shutdown before the application.

See also:
    https://groups.google.com/forum/#!msg/play-framework/HPIgp2TTVtE/kkwerzFyVUkJ
    http://play.lighthouseapp.com/projects/82401/tickets/632-stop-plugins-in-reverse-order-that-they-were-started
    https://github.com/playframework/Play20/pull/397