v2.2.6

Adds tag filtering for task listings and fixes a race in the task database plugin.

• Task listings can be filtered by tag: the GET /tasks endpoint and the ls
  command of the task CLI
  accept a repeatable tags option that returns only tasks carrying at least
  one of the given tags, and TaskInfo now reports each task's tags.
  (#94)
• The task database plugin
  now serialises its per-run lifecycle writes with a dedicated task-run lock.
  CrudDB.db_upsert is not atomic — it issues an UPDATE and only INSERTs
  when nothing matched — so when the scheduler wrote the queued row and a
  consumer wrote the running row a few milliseconds later, the consumer's
  UPDATE could miss the not-yet-committed INSERT, fall through to its own
  INSERT and violate the task-runs primary key. Holding the lock around the
  upsert removes the race.
  (#95)
• TaskRun.lock
  accepts an optional name to acquire a named sub-lock for the task run, and
  timeout now defaults to None. (#95)