DCP client doesn't cancel gocbcore open stream op on timeout

When an open stream request times out a timeout error is immediately returned with no other actions taken. Operations in gocbcore return a PendingOp which allows cancellation of the operation. If the operation is not cancelled then it will remain in the system, and could cause unwanted side effects.

For example if a slow operation is timed out but remains in the system then a response may still come in which will call the registered handler. If this handler ends up blocking for some reason, because nothing is expecting it to be called, then it could block the agent read thread.