[Elastic Agent] When GRPC connected and Stop occurs, a disconnect should be considered stopped

[blakerouse]

Version: Elastic Agent 7.9 BC2

Currently Elastic Agent has a strict contract for GRPC stopping action. The flow must follow exactly this:

1. Send stopping to client
2. Receive stopping from client
3. Client disconnects

In some cases if the client is quick to send stopping and disconnect, the socket will not flush and the server will not get the stopping message. To help with this, Elastic Agent should be more relaxed. It should also check if the client is currently connected and Stop is performed and the client just disconnects consider that a valid completed stop.