Fixes#12560
When gracefully disconnecting from the coordinator, we would send the Disconnect message and then close the dRPC stream. However, closing the dRPC stream can cause the server not to process the Disconnect message, since we use the stream context in a `select` while sending it to the coordinator.
This is a product bug uncovered by the flake, and probably results in us failing graceful disconnect some minority of the time.
Instead, the `remoteCoordination` (and `inMemoryCoordination` for consistency) should send the Disconnect message and then wait for the coordinator to hang up (on some graceful disconnect timer, in the form of a context).
When an agent receives a node, it responds with an ACK which is relayed
to the client. After the client receives the ACK, it's allowed to begin
pinging.
