Time based batching aggregation

Currently batching is using the following heuristic to decide when an aggregation action should be executed: aggregate all outstanding batchable tasks when plan queue is empty and there is no task that can make immediate progress. While this is intended behavior in many situations, sometimes different mechanism is required e.g. time based.
This task is to consider and implement time based batching aggregation.

Start by tracing the existing batching aggregation heuristic and the code paths that decide when aggregation runs. Review any related tests and determine the expected timing behavior, triggering conditions, and completion criteria before designing an implementation.

Written by the indexing model from the issue text.