chore(db-scripts): remove foreign key constraint on SUPER_CASE_EXEC_

This commit removes an unnecessary foreign key constraint on column
SUPER_CASE_EXEC_ inside the db table ACT_RU_CASE_EXECUTION.

If a CaseTask is non-blocking then the associated case execution will be
completed after the instantiation of the called case instance. During the
completion of the CaseTask the associated case execution will be removed
and with the foreign key constraint this would not be possible, because
the referential integrity would not be satisfied.

related to CAM-2444
feat(case/caseTask): execute a case task

This commit provides the implementation of the CaseTask.

If a case task becomes "ACTIVE" a given case will be instantiated,
e.g:

  <caseTask id="anId" caseRef="aCaseToInstantiate" ... />

In that case a new case instance will be started by the given case
definition key ("aCaseToInstantiate").

Furthermore it is possible to declare one of the following
"caseBinding":

- latest: means that a case instance of the latest version of the
  given case definition will be instantiated.
- deployment: means that a case instance of the case definition
  which is inside the same deployment as the case definition will be
  instantiated.
- version: means that a case instance of a declared
  "caseVersion" will be instantiated (therefore you need to provide
  a further attribute called "caseVersion").

Additionally it is possible to declare which variables should be exchanged
between the case instances (and vice versa), e.g.:

  <caseTask ... >
     <extensionElements>
       <camunda:in source="a" target="b" />
       <camunda:out source="b" target="c" />
     </extensionElements>
  </caseTask>

And the BusinessKey like this:

  <caseTask ... >
    <extensionElements>
      <camunda:in businessKey="${caseExecution.caseInstanceBusinessKey} />
    </extensionElements>
  </caseTask>

related to CAM-2444