Upgrading Openfire to use Jetty 7 Continuations

See http://www.igniterealtime.org/community/message/194789#194789

One advantage of upgrading is that while jetty-6-style continuations were a Jetty-specific mechanism, the jetty-7-style continuations are portable and work with a variety of containers. This means that they'll work asynchronously with Jetty 6/7/8 and any servlet-3.0 containers. They're also still compatible with servlet-2.5 containers, where they'll be blocking.