Delete byte array only if there is one

# By Tijs Rademakers
# Via Tijs Rademakers
* 'activiti6' of https://github.com/Activiti/Activiti:
  First stage of job executor backwards compatibilty

Breaking change:  org.activiti.engine.impl.pvm.delegate; (ActivityBehavior and others) changed to  org.activiti.engine.impl.delegate;

Delete byte array only if there is one, as VariableInstanceEntity.getByteArrayRef never seems to return null

Removing ReadonlyProcessDefinition interface. PVM heritage + all methods were already in ProcessDefinition.

All compile errors gone! Had to comment a bit of Crystall Ball code, which was not used in any test it seems (todo: verify with Martin), as the AbstractSimulationActivityBehavior does not have any subclass.

Removing activiti-diagram-rest module. This will be replaced by functionality in the new Activiti 6 UI

Addition CDI fixes. Made the ExecutionContext CDI only, which means that for non-CDI usage that thread local is not used (yay, microperformance).

But MOST importantly: the -Pcheck now runs green (at least on H2)!

Fixing up CDI multi instance test: making sequential MI is called consistently like parallel MI (ie. instances + 1)

Fix for SQL server failing with 'Prepared or callable statement has more than 2000 parameter markers' : added a configuration param that sets the max nr of insert statements in one bulk insert.

Manual merge of https://github.com/Activiti/Activiti/commit/a0c4a1881b5775b5156e9be9d29e099ac8a8afca to v6: still got to leave the optimize delete in here, as it optimizes the delete for execution (the order, which is quite important and cannot be deleted yet)

Manual merge of https://github.com/Activiti/Activiti/commit/60ae72ed924fc6426de0996e8ecdd2fd0a8de003 to v6