v0.21.0

• Worker's page on the dashboard now shows worker tags and a badge if the
  worker is the cluster's current leader.
• The workflow.upserted event has been split into workflow.created and
  workflow.updated events. Added a new workflow.step_completed event.
• WorkflowPlugin.push now only pushes the workflow if it has actually changed.
• workflow.step_completed is now used to nearly instantly progress a workflow
  to the next step, instead of waiting for the next polling interval.
• The WorkflowPlugin now exposes _ex versions of most functions which touch
  the database, allowing you to pass in your own cursor to ensure that a
  workflow is created in the same transaction as other operations.
• The WorkflowPlugin now locks the workflow row when updating it and performs
  all updates in a single transaction.
• All _ex functions which take a cursor now expect a cursor using a dict_row
  row factory.
• Improved metrics display by adding automatic formatting for large numbers
  (K, M, G) on chart Y-axes.
• Added get_schedules() helper method to the Cron plugin to retrieve
  scheduled jobs.
• Added a live stream of the internal events queue to the dashboard.
• Table size metrics are now collected for all tables known to plugins.