Cmmn: fix end time of historic task not always set + identitylinks not cleaned up after test + sql server upgrade fix

Refactor the way the plan model is represented at runtime: from being a planItemInstance to implicit within the caseInstance, which makes the queries more consistent

Terminating case instance (part 2) : nested stages + dedicated behavior for transitions through PlanItemActivityBehavior

Added CaseInstance, PlanItemInstance and MilestoneInstance (similar to process engine concepts) and basic agenda operation to activate plan items of case model.

Refactoring CommandContext usage: centralizing CommandContext and related classes (interceptor, transaction handling, dbSqlSession, ...) in flowable-engine-common module.
Updated the engine configurators to reuse as much as possible from the process engine when used together.
This is a work in progress: some more fine-tuning needed + docs need to be updated where applicable.

* JavaDoc, add license, misc. spelling, punctuation, etc changes

* JavaDoc, add license, misc. spelling, punctuation, etc changes

* JavaDoc, add license, misc. spelling, punctuation, etc changes

* JavaDoc, add license, misc. spelling, punctuation, etc changes

Format back to 2 spaces for now, merging back from Activiti 5 is really an issue with spacing differences

First cut of DatabaseEventLogger: alternative for history where the event mechanism is used to generate database events (as json, so they can easily be fed into a json NoSQL store). This feature is turned off by default, as its experimental.