Add heartbeat / automatic server cleanup code

This replaces the previous method of attempting to clean up servers when
an unexpected exit occurs in the client (e.g. SIGINT or panic) by a
heartbeat protocol.  If the server does not hear from the connecting
client within a certain interval of time (500ms in this commit), it will
de-activate itself.  This prevents dangling Docker Machine server
processes from accumulating.

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>

libmachine/drivers/plugin/localbinary/plugin_test.go

@@ -106,7 +106,7 @@ func TestExecServer(t *testing.T) {
 		stopCh:      make(chan bool, 1),
 	}
 
-	finalErr := make(chan error, 1)
+	finalErr := make(chan error)
 
 	// Start the docker-machine-foo plugin server
 	go func() {