Avoid resetting Websocket idle_timeout timer too often

`perf` has shown that Cowboy spends a lot of time
cancelling and starting this timer. Instead of resetting
for every data received, we now only reset a field in the
state.

Before it was working like this:

- start idle timeout timer
- on trigger, close the connection
- on data, cancel and start again

Now it's working like this:

- start idle timeout timer for a tenth of its duration, with tick number = 0
- on trigger, if tick number != 10
  - start the timer again, again for a tenth of its duration
  - increment tick number
- on trigger, if tick number = 10
  - close the connection
- on data, set tick number to 0

Makefile

@@ -9,6 +9,7 @@ PROJECT_REGISTERED = cowboy_clock
 
 PLT_APPS = public_key ssl # ct_helper gun common_test inets
 CT_OPTS += -ct_hooks cowboy_ct_hook [] # -boot start_sasl
+#CT_OPTS += +JPperf true +S 1
 
 # Dependencies.