data store: add is_retry and is_held indicators

[oliver-sanders]

This is related to the redesign/rethink of the workflows sidebar in cylc-ui. See cylc/cylc-ui#2331, cylc/cylc-ui#2350

The data store maintains some task status information at the workflow level which is used to populate the workflows sidebar in the Cylc GUI.

Currently we have this:

• Task state totals
• is_held totals
• is_queued totals
• is_runahead totals

With the redesign of the workflows sidebar, we require:

• Task state totals
• is_held indicator
• is_retry indicator

Note, we only require True/False indicators, not counts for the is_held and is_retry fields.

Suggest adding the two new fields, and potentially deprecating the remaining totals fields (unused?).