WorkflowKernel - do not clone the workitem instance

[rsoika]

Currently the WorkflowKernel clones the workitem before it will be processed by plugins and adapters. This blocks the possibility for a plugin to update some data and throw a pluginException. But this can be very helpful for complex input validation where a plugin want to provide an additional error context.

So in the future we will no longer do a deep copy of the workitem in the WorkflowKernel.