OPENDJ-830 (CR-1554) Implement authentication and authorization for HTTP connection handler


Improved error reporting to the client: Internal exceptions are now returned to the client as such.
This simplified the error handling code.


CollectClientConnectionsFilter.java:
Renamed sendUnauthorizedResponseWithHTTPBasicAuthChallenge() to sendErrorResponse().
Extracted more generic method toJSON() from sendErrorResponse().
Removed several try / catch + logging and replaced them with throwing the exception up.

HTTPClientConnection.java:
A connection is now valid by default (doh!)

CollectClientConnectionsFilterTest.java:
Updated the tests.