Rex::Output does not persist across different processes (forks)

This affects batches. For instance, given 'tasks_batch' batch exists and
cosisted of several tasks, so running:
rex -o JUnit -b tasks_batch

will create a junit output only for the first taks, then
Rex::Output::JUnit will be collected via the GC which in turn will casue
any following calls to it will fail / wont write to the file.

using IPC::Shareable to share the object across processes allows the
aggregations of all the tasks into a single JUnit xml file which is the
expected behavior.