data_store_mgr: clear data from workflows that no longer have active subscriptions

[oliver-sanders]

We need the top-level workflow data from every workflow in a user's ~/cylc-run directory in the UIS data store, e.g:

• Workflow ID
• Host
• Port
• etc

Additionally, we require any other objects that the user has requested, e.g:

• Tasks
• Jobs
• Graph nodes/edges
• etc

When a user navigates away from a workflow, we no longer need to preserve these Tasks, Jobs, etc in the store and should purge it in due course to avoid stopped workflow data accumulating.

One approach would be to set a timer when the last subscription for a workflow is closed [1]. Unless another subscription on the workflow [1] is opened when this timer expires, then we purge the data.

This issue is similar to #568 which is about reducing the number of Protobuf "topics" we subscribe to as this issue also requires the monitoring of active subscriptions. This issue differs in that it is about clearing the data in the store that we have already acquired but is no longer of use rather than reducing the amount we request.

[1] Excluding top-level only subscriptions (see above) as this data is always required.