Multi instance refactoring: introduced the 'IS_MI_ROOT_' column to indicate an execution is the root of a multi instance. Every multi instance (sequential and parallel, parallel already had it) will now spawn an execution that acts as parent for the whole multi instance. This has two benefits: cleanup becomes way easier and it opens up the possibility to dynamically add new instances to a multi instance (not yet implemented).

Porting 'terminateAll' to v6: cherry pick of https://github.com/Activiti/Activiti/commit/0727dd10c86cbb50089979289e300c6998a159b2

Implement ClassDelegateFactory and DefaultClassDelegateFactory, and use them in the DefaultActivityBehaviorFactory and the DefaultListenerFactory.

ACT-4079 (NullPointerException in DbSqlSessionFactory.isBulkInsertable() when Custom Entity is inserted) : fixing Oracle, the old 'get()' needs to be back, but now _after_ the containsKey

Rename ClassDelegate.instantiateDelegate to ClassDelegate.defaultInstantiateDelegate for external class, and added a non-static method for internal calls.

Multitenant process engine configuration with isolated schema for each tenant (with dynamic tenant registration).
+ Two implementations of multi tenant (isolated schema) async executors: one with an executor/tenant, the other one with a shared executor service but separate acquisition threads

Removed TaskDefinition: finally got rid of this bad idea of a class. Switched the implementations using it to determine it at runtime (consitent with all the rest).

Massively Cleanup Up TaskEntity: making it what it should be, a container for data and not a central hub for all kinds of operations (slightly overreacted).

Passing the processEngineConfigurationImpl to the entity/data managers on instantiation instead of relying on the Context lookup.

Refactored all entity classes: added an interface for each entity, such that alternative data stores can swap out the entity objects with more fitting ones (eg with annotations like Neo4j, JPA, etc or other mechanisms. Internally, only the entity interface is used)

Fix for failing Oracle QA

Refactoring the persistence: splitting of persistence specific code in the EntityManager classes into a DataManager class.
The EntityManager -> EntityManagerImpl  now each have a dedicated DataManagerImpl that contains all the MyBatis specific code,
effectively making it possible to plug that layer with some other implementation.

Note: Still need to check the usages of DbSqlSession in the behaviors/commands, but those should be easily portable too.