User story

When fetching and ranking ext. tasks by priority, I want to use the creation time of the task as a secondary sorting. This ensures that no external tasks starve in high load situations. The engine has already a similar feature for job execution: https://docs.camunda.org/manual/7.14/user-guide/process-engine/the-job-executor/#the-job-order-of-job-acquisition 

Functional Requirements (Required before implementation):

• For the fetch and lock request (Java and REST API), there is a new parameter to toggle ordering by creation time
• If priority is also used, the creation time ordering is the secondary ordering
• If priority is not used, the creation time ordering is the primary ordering
• If the creation time parameter is not set, the behavior is the same as it is now (i.e. undefined with respect to creation time)

Technical Requirements (Required before implementation):

• Creation time of tasks is stored in the external task runtime table
• Evaluate if we should new indexes to the schema (see also https://docs.camunda.org/manual/7.14/user-guide/process-engine/the-job-executor/#the-job-order-of-job-acquisition for how this is solved for jobs)

Limitations of Scope (Optional):

Hints (Optional):