feat(http2): add HTTP2 keep-alive support for client and server

This adds HTTP2 keep-alive support to client and server connections
based losely on GRPC keep-alive. When enabled, after no data has been
received for some configured interval, an HTTP2 PING frame is sent. If
the PING is not acknowledged with a configured timeout, the connection
is closed.

Clients have an additional option to enable keep-alive while the
connection is otherwise idle. When disabled, keep-alive PINGs are only
used while there are open request/response streams. If enabled, PINGs
are sent even when there are no active streams.

For now, since these features use `tokio::time::Delay`, the `runtime`
cargo feature is required to use them.

src/proto/h2/mod.rs

@@ -12,8 +12,8 @@ use crate::body::Payload;
 use crate::common::{task, Future, Pin, Poll};
 use crate::headers::content_length_parse_all;
 
-pub(crate) mod bdp;
 pub(crate) mod client;
+pub(crate) mod ping;
 pub(crate) mod server;
 
 pub(crate) use self::client::ClientTask;