Automatically scale up worker versions with reachability ClosedOnly on query

[jlegrone]

Right now it is hard to scale to zero worker versions that only have closed workflows, while still detecting that queries are being sent to those versions and scaling back up when necessary.

The TaskAddRate statistic from the DescribeTaskQueue API does include query tasks, but is not guaranteed to catch sporadic queries (eg. due to a human debugging a single closed workflow execution).

[carlydf]

The newly added SunsetStrategy is intended to help address this by allowing users to specify a window during which they expect to query closed workflows in a version. It still does not cover unexpected queries that come after that window.

[jlegrone]

Related to #52