[API] Introduce replaceable XStream converters

Since XStream has no built-in mechanism for unregistering converters,
this patch introduces a mechanism to deactivate and replace registered
XStream converters. This allows converters to rely on certain states,
such as a running Saros session.

Change-Id: Ibd26fbf4af3fb4400c78e79cda18faf39359a428
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1519
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>