I have the same issue I found the source of LocalHTWorkItemHandler here :
https://github.com/droolsjbpm/jbpm/tree/master/jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/process/workitem/wsht
unfortunately it's version 5.4.0-SNAPSHOT of jbpm I don't think it's included in 5.3.final