Backport the dispatcher thread from jBPM 4 to avoid race conditions with multiple JobExecutor threads

Running two or more instances of the following test process leads to deadlock situations in MySQL when multiple JobExecutor threads are active:

<process-definition xmlns="urn:jbpm.org:jpdl-3.2" name="Process_Timer2">
<start-state name="start-state1">
<transition to="node1"></transition>
</start-state>
<state name="Sleep">
<timer duedate="1 seconds" name="sleep_timer_2" transition="OK" repeat="NO">
<action></action>
</timer>
<transition to="node1" name="OK"></transition>
</state>
<node name="node1">
<action class="Wait"></action>
<transition to="Sleep"></transition>
</node>
</process-definition>

The Wait class does nothing more than sleep for a second before continuing the execution:

public void execute(ExecutionContext executionContext) throws Exception

{ Thread.sleep(1000); executionContext.leaveNode(); }