[CX] disconnect, connect link slow with large number of shadows

On my laptop, disconnecting a link w/ 200 shadows (each containing 1 mutation) takes over a minute, and about half that to subsequently reconnect. This will result in a pretty abysmal customer experience when executing a DDL which currently implicitly executes a disconnect + connect cycle.

We should consider disconnect / connect at the collection (as opposed to link) level; e.g. alter collection enable analytics could avoid disconnect completely; alter collection disable analytics could maintain the ingestion pipeline, while discontinuing ingestion for the affected collection/dataset.