feat(http2): Make HTTP/2 support an optional feature

cc #2251

BREAKING CHANGE: This puts all HTTP/2 methods and support behind an
  `http2` cargo feature, which will not be enabled by default. To use
  HTTP/2, add `features = ["http2"]` to the hyper dependency in your
  `Cargo.toml`.

src/proto/h1/date.rs

@@ -3,6 +3,7 @@ use std::fmt::{self, Write};
 use std::str;
 use std::time::{Duration, SystemTime};
 
+#[cfg(feature = "http2")]
 use http::header::HeaderValue;
 use httpdate::HttpDate;
 
@@ -21,6 +22,7 @@ pub fn update() {
     })
 }
 
+#[cfg(feature = "http2")]
 pub(crate) fn update_and_header_value() -> HeaderValue {
     CACHED.with(|cache| {
         let mut cache = cache.borrow_mut();

src/proto/h2/server.rs

@@ -10,7 +10,7 @@ use tokio::io::{AsyncRead, AsyncWrite};
 
 use super::{decode_content_length, ping, PipeToSendStream, SendBuf};
 use crate::body::HttpBody;
-use crate::common::exec::H2Exec;
+use crate::common::exec::ConnStreamExec;
 use crate::common::{task, Future, Pin, Poll};
 use crate::headers;
 use crate::proto::Dispatched;
@@ -95,7 +95,7 @@ where
     S: HttpService<Body, ResBody = B>,
     S::Error: Into<Box<dyn StdError + Send + Sync>>,
     B: HttpBody + 'static,
-    E: H2Exec<S::Future, B>,
+    E: ConnStreamExec<S::Future, B>,
 {
     pub(crate) fn new(io: T, service: S, config: &Config, exec: E) -> Server<T, S, B, E> {
         let mut builder = h2::server::Builder::default();
@@ -162,7 +162,7 @@ where
     S: HttpService<Body, ResBody = B>,
     S::Error: Into<Box<dyn StdError + Send + Sync>>,
     B: HttpBody + 'static,
-    E: H2Exec<S::Future, B>,
+    E: ConnStreamExec<S::Future, B>,
 {
     type Output = crate::Result<Dispatched>;
 
@@ -216,7 +216,7 @@ where
     where
         S: HttpService<Body, ResBody = B>,
         S::Error: Into<Box<dyn StdError + Send + Sync>>,
-        E: H2Exec<S::Future, B>,
+        E: ConnStreamExec<S::Future, B>,
     {
         if self.closing.is_none() {
             loop {

src/proto/mod.rs

@@ -5,7 +5,9 @@ pub(crate) use self::body_length::DecodedLength;
 pub(crate) use self::h1::{dispatch, Conn, ServerTransaction};
 
 pub(crate) mod h1;
-pub(crate) mod h2;
+cfg_http2! {
+    pub(crate) mod h2;
+}
 
 /// An Incoming Message head. Includes request/status line, and headers.
 #[derive(Clone, Debug, Default, PartialEq)]