This flag is set when RM receives the response message for this async
invoke. This flag is checked in isExpired to prevent the async invoke state
from getting deleted until the pending async response is processed.
Without this flag, only the offer sequence is checked, and it can close
between the time that we receive the async response, and the ack goes back
to the sender, and when the WsDispatchMessageListener actually picks up
the response and processes it.