Use a generic Timer to implement the timeouts and intervals

[seanmonstar]

After adding a generic T: Timer, update the uses of tokio::time to use the generic timer instead.

• HTTP/2 keep-alive interval
• HTTP/2 keep-alive timeout
• HTTP/1 read headers timeout

[Robert-Cunningham]

I'll look to take on the timer series of issues (#2847, #2846, #2848, #2857).