stats: retain Scopes for some amount of time after the last reference to them is dropped, to avoid leaking data when xds removes a scope

*Title*: When an xDS update removes a cluster, any stats that get updated after the most recent sink or scrape will get dropped when the ScopeSharedPtr held in the cluster gets dropped.

*Description*:
>To avoid this we should delay the destruction of the Scope for some time interval, perhaps a few minutes. We must be careful to accelerate the Scope destruction when the stats store gets destructed (server exit, or unit-test completion).