+++ This bug was initially created as a clone of Bug #1245616 +++

Description of problem:
When PerProcessInstance strategy is used and GetProcessInstanceByCorrelationKeyCommand is sent to /execute endpoint via REST then following exception is raised:

 [org.drools.persistence.SingleSessionCommandService] (http-/127.0.0.1:8080-3) Could not commit session: java.lang.IllegalStateException: Invalid session was used for this context org.kie.internal.runtime.manager.context.ProcessInstanceIdContext@79f8494f

Version-Release number of selected component (if applicable):
6.1.1

How reproducible:

always
Steps to Reproduce:
1. Start any process StartCorrelatedProcessCommand 
2. Try to get this process with GetProcessInstanceByCorrelationKeyCommand
Process definition can be the simplest one.

I have used following exact commands (sent to /execute endpoint):
https://gist.github.com/anonymous/36173d7009da03b46f55

Actual results:
Exception is raised when GetProcessInstanceByCorrelationKeyCommand is used via REST with PerProcessInstance strategy

Expected results:
Corresponding process should be correctly returned when GetProcessInstanceByCorrelationKeyCommand is used via REST API

Additional info:
This may be caused by the fact, that behind the scenes the wrong context is used for obtaining the RuntimeEngine, i.e. maybe something like this:

    manager.getRuntimeEngine(ProcessInstanceIdContext.get())

while this one should be used instead:

manager.getRuntimeEngine(CorrelationKeyContext.get(key)) when CorrelationKey related commands are used.