feat(h2): use a more reasonable timer for keep alive timeout in h2

[joelwurtz]

I was looking at why the keep alive h2 test was running for so long (5 sec)
and this was the * 10 mult on duration that was causing this.

I'm not sure this a good default, and maybe it should be configurable, in fact in h2, it may need 2 different duration configurable:

• keep_alive_idle_timeout : which is the duration to wait to launch keep alive (ping pong) check after no more requests are received (it's the current value of keep_alive_timeout)
• keep_alive_timeout : which is the duration to wait for a ping pong response from the client (currently equal to keep_alive_timeout * 10 - keep_alive_timeout)

[fakeshadow]

There is some behavior change in xitca-http 0.9.0 where the ping-pong is running in a fixed interval instead of only when idle. I would suggest using a higher value as default if the default 5 seconds is too harsh.

I would push a fix later to resort the behavior to prior 0.8