VAPID header caching

[javiermarinros]

As @ozgurhangisi suggested in #195

Just enabling it with $webPush->setReuseVAPIDHeaders(true); does the trick. Tested in my production environment.

[Minishlink]

Looks good! :) Can you mention this in the doc and indicate that it is a tradeoff between security and perf?

[javiermarinros]

Done. I would like to point that there is barely no security tradeoff, as the only payload data that changes between invocations to getVapidHeaders() is the expiration timestamp. Maybe this setting could be enabled by default or extend it longer than the flush session (e.g. using the current hour in the cache key, so VAPID headers would be renewed every hour)

[Minishlink]

Thanks!