Offer separate KeepAlive directive for h2

[icing]

The default http/1.1 KeepAlive setting in 5 seconds, which is too penalizing for h2 clients (they only use a single connection to a domain, so the expectation would be that longer keepalives will still burden the server less than h1)

Since httpd parks idle connections only for async mpms, the default for h2 on such a mpm should be higher by default. It could work like this:

H2KeepAlive "KeepAlive"|n

With a default of max(KeepAlive,60) on async mpms and equal to KeepAlive otherwise.

[icing]

Additionally, that value could be announced to clients in a KeepAlive header.