add storage interface to store old user tasks executions

[abhishekmendhekar]

The following PR #304 adds support for UserTasks that enables tracking of active (async) user operations.

UserTasksManager maintains all old tasks in-memory for 6 hours before discarding all the completed tasks but by adding a storage interface these operations can be stored persistently. In the future, additional APIs can be added or direct calls to the store (could be SQL) to get historical data for analysis.

Ref #293

[efeg]

As of today, the user_tasks endpoint returns the summary of recently completed and active tasks from various endpoints such as rebalance, remove_broker, and state. Using the relevant UUID in the header of the corresponding request, users can retrieve the response of a completed task.

The duration to keep such completed responses in memory is now controlled by two configurations: (1) completed.user.task.retention.time.ms (current: 6 hours) and (2) max.cached.completed.user.tasks (indirectly). The need for an API to store the historical data in a persistent store still exists.