fix(http2): gate server Builder keep-alive interfaces

[cratelyn]

currently, dependents of hyper 0.14 that enable the server and http2 feature flags will see an error if they enable the backports flag.

building hyper like so...

cargo check --features server,http2,backports

...will yield the following errors (shown in "short" format here for brevity):

src/server/conn.rs:75:9: warning: unused import: `tracing::trace`
src/server/conn/http2.rs:185:25: error[E0609]: no field `keep_alive_interval` on type `h2::server::Config`
src/server/conn/http2.rs:199:25: error[E0609]: no field `keep_alive_timeout` on type `h2::server::Config`
warning: `hyper` (lib) generated 1 warning
error: could not compile `hyper` (lib) due to 2 previous errors; 1 warning emitted

this stems from the fact that the deprecated connection builder hyper::server::conn::Builder and the backported
hyper::server::conn::http2::Builder both operate on the same internal Config structure. this structure has changed slightly between 0.14 and 1.0 however. see:

; git diff 0.14.x master -- src/proto/h2/server.rs | grep 'struct Config' -A 11

@@ -51,12 +51,11 @@ pub(crate) struct Config {
     pub(crate) max_concurrent_streams: Option<u32>,
     pub(crate) max_pending_accept_reset_streams: Option<usize>,
     pub(crate) max_local_error_reset_streams: Option<usize>,
-    #[cfg(feature = "runtime")]
     pub(crate) keep_alive_interval: Option<Duration>,
-    #[cfg(feature = "runtime")]
     pub(crate) keep_alive_timeout: Duration,
     pub(crate) max_send_buffer_size: usize,
     pub(crate) max_header_list_size: u32,
+    pub(crate) date_header: bool,
 }

the runtime feature flag has since been removed. see: https://docs.rs/hyper/1.5.1/hyper/#optional-features

the backported code, consequently, does not include these conditional compilation attributes. i was able to recreate this as far back as v0.14.25, when these backported types were first released.

this commit proposes the addition of #[cfg(feature = "runtime")] attributes to the keep_alive_timeout(..) and keep_alive_interval(..) methods of the backported http/2 server connection Builder.

this will allow hyper users that only require the server and http2 feature flags to address deprecations and prepare to upgrade to hyper 1.0, without having to opt into previously disabled feature flags.

[seanmonstar]

Thanks for the write-up and fix!