Adjust naming scheme for session specific timeouts

This arose in issue #1663 over a discussion of settings proxy.config.http2.accept_no_activity_timeout and proxy.config.http2.no_activity_timeout_in. It was pointed out that really these settings control timeouts on the H2 session much like the keep_alive settings do for HTTP/1.x. And it is likely that future protocols like Http/3 will have the need for a similar knob.

It will be useful to adjust these setting on a per protocol basis. Perhaps you want to let HTTP/2 session be idle longer than HTTP/1 sessions since there are likely more HTTP/1.x sessions.

Originally we talked about renaming them to keep_alive settings to line up with the HTTP/1.x version of the settings, but
@maskit pointed out that the keep_alive concept is very HTTP/1.1 specific, and instead he suggested using session_no_activity_timeout_in/out naming scheme. So for HTTP/2 we would have proxy..config.http2.session_no_activity_timeout_in(and _out).