SHIELD v8.6.1

This release chiefly introduces logic to existing and new database
schema migrations to fix fallout from the 8.6.0 release, in which
we mistakenly modified a historic migration to do something.

All database upgrade paths, including pre-8.6.0 → present,
(failing) 8.6.0 deployment → present, and brand new deployments,
should all work now.

Improvements

• The fs plugin is now quieter by default, and will only turn on
  per-file debug logging if asked to do so via its own
  configuration. This should greatly speed up backup operations
  on busy SHIELDs, since it reduces the database lock contention.

Bug Fixes

• The front-end configuration wizard now properly looks up
  plugin metadata for an agent. Previously, there was a
  Javascript variable shadowing bug that caused the front-end to
  return any arbitrary plugin metadata as the "correct" one.

• Task cancelation had a n inverted boolean assertion on global
  tenant-iness that has been fixed.