Verifies that a multi instance with a large input collection can activate all it's
 children without running out of batch size.
 The test also verifies that batching is used for the activating.

Improves the assertion message by explaining what the next steps are if it fails.

Adds the ActivateProcessInstanceBatchProcessor and registers it with the Engine.

This processor is responsible to handle the ProcessInstanceBatch.ACTIVATE commands.
It is used for activating child element instance of a multi instance element in a batch-like manner.
The index in the command is used to indicate how much children need to be activated.
If the amount of children will exceed the max message size a followup batch command is written.
The index of the followup command is decremented for each of the activate commands that have been send.

A batch ACTIVATE operation works different from a TERMINATE batch operation.
During a TERMINATE the index is the key of a child element instance. For ACTIVATE it is a simple counter,
 keeping track of how many ACTIVATE commands we still need to write.

Instead of writing all the ACTIVATE commands for all children at once, we now use the
ProcessInstanceBatch.ACTIVATE command instead.

This command takes an index, which in this case is the amount of children that an ACTIVATE command will be written for.