Provide information about variables changed during process instance modification

[zambrovski]

Scenario

As a developer, I want to get the state of current process variables assigned to the executions of a process instance. For example, if decision evaluation requires a set of variables as inputs, I want to re-valuate this decision only if one of those variables changes.

The feature should provide a process instance store (cache). I want to be able to subscribe to this cache on changes of a certain process instance id (or one of the linked execution ids). Optionally, I want to pass a list of variable names (and their scopes) I'm interested in.