feat(http1): Add higher-level HTTP upgrade support to Client and Server (#1563)

- Adds `Body::on_upgrade()` that returns an `OnUpgrade` future.
- Adds `hyper::upgrade` module containing types for dealing with
  upgrades.
- Adds `server::conn::Connection::with_upgrades()` method to enable
  these upgrades when using lower-level API (because of a missing
  `Send` bound on the transport generic).
- Client connections are automatically enabled.
- Optimizes request parsing, to make up for extra work to look for
  upgrade requests.
  - Returns a smaller `DecodedLength` type instead of the fatter
    `Decoder`, which should also allow a couple fewer branches.
  - Removes the `Decode::Ignore` wrapper enum, and instead ignoring
    1xx responses is handled directly in the response parsing code.

Ref #1563 

Closes #1395

src/client/mod.rs

@@ -268,7 +268,7 @@ where C: Connect + Sync + 'static,
                                 .h1_writev(h1_writev)
                                 .h1_title_case_headers(h1_title_case_headers)
                                 .http2_only(pool_key.1 == Ver::Http2)
-                                .handshake_no_upgrades(io)
+                                .handshake(io)
                                 .and_then(move |(tx, conn)| {
                                     executor.execute(conn.map_err(|e| {
                                         debug!("client connection error: {}", e)