[INTERNAL] moved proxy resolving logic out of ConnectionHandler

The ConnectionHandler class should be part of the Core. Currently it was
not possible to move this class because of the Eclipse stuff that
resolved the proxy settings.

Change-Id: If910c1af09ae57f61d0f4cb38eef54ab20becba3
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1807
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>