OF-1326: Improve sharing of the BOSH context

Intead of using a parent context, a collection of Jetty handlers can be used to dynamically add/remove functionality to the BOSH context.

This commit replaces the parent context instance with a ordered list of handlers, which will attempt to process a request by:

1. Checking if this is a BOSH request
2. Checking if this is a request for BOSH metadata
3. Check if an extension was provided that can handle the request
4. Try to serve static content as a last resort.

In step 3, a collection of handlers is used, that can be modified at runtime. This allows plugins to register/remove handlers.

The entire collection of handlers (1 to 4) is maintained with a lifecycle that's different from the embedded Jetty server that uses them. This
allows the collection to survive a server reconfiguration, as well as act independent of the 'enabled' state of the BOSH service.