client: add `Subscription::close_reason` (#1320)

* client subscription: don't close subscription lagg

* refactor(client subscription): don't drop on lagg.

* add client subscription example

* Update core/src/client/mod.rs

* Update examples/examples/client_subscription_drop_oldest_item.rs

* address grumbles

* Update tests/tests/integration_tests.rs

* remove async-broadcast dep

* revert subscription tests

* Update core/src/client/mod.rs