HTTP/2 stream handling fixes.

This address some issues shinrich found in prod testing. The biggest
changes are:

1. HTTP/2 now correctly sends body data if there is no Content-Length
header.

2. The mechanism by which HTTP/2 streams are handled is chanced such
that entire streams are handled at once rather than trying to separate
the read of headers vs body. The latter doesn't work if you get headers
for stream id 1, headers for stream id 3, then body for stream id 1, for
instance.

3. This also fixes poll_for_headers for HTTP/2 so that if requests are
read on a previous call, this will not block indefinitely.

local/include/core/ProxyVerifier.h

@@ -1786,11 +1786,11 @@ class H2StreamState
    */
   swoc::TextView register_rcbuf(nghttp2_rcbuf *rcbuf);
 
-  /** Indicate that the stream has ended. */
-  void set_stream_has_ended();
+  /** Indicate that the stream has closed. */
+  void set_stream_has_closed();
 
-  /** Return whether this stream has ended. */
-  bool get_stream_has_ended() const;
+  /** Return whether this stream has closed. */
+  bool get_stream_has_closed() const;
 
   /** Set the stream_id for this and the appropriate members. */
   void set_stream_id(int32_t id);
@@ -1822,7 +1822,7 @@ class H2StreamState
 private:
   int32_t _stream_id = -1;
   std::deque<nghttp2_rcbuf *> _rcbufs_to_free;
-  bool _stream_has_ended = false;
+  bool _stream_has_closed = false;
   nghttp2_nv *_request_nv_headers = nullptr;
   nghttp2_nv *_response_nv_headers = nullptr;
 };
@@ -1838,6 +1838,12 @@ class H2Session : public TLSSession
   swoc::Rv<ssize_t> write(swoc::TextView data) override;
   swoc::Rv<ssize_t> write(HttpHeader const &hdr) override;
 
+  /** For HTTP/2, we read on the socket until an entire stream is done.
+   *
+   * For HTTP/1, we first read headers to get the Content-Length or other
+   * header information to direct reading the body. For HTTP/2, this isn't
+   * an issue because bodies are explicitly framed.
+   */
   swoc::Rv<int> poll_for_headers(std::chrono::milliseconds timeout) override;
   swoc::Rv<std::shared_ptr<HttpHeader>> read_and_parse_request(swoc::FixedBufferWriter &w) override;
   swoc::Rv<size_t> drain_body(
@@ -1864,15 +1870,15 @@ class H2Session : public TLSSession
     return _session;
   }
 
-  /** Indicate that a stream has completed its HEADERS frame.
+  /** Indicate that the stream has ended (received the END_STREAM flag).
    *
-   * @param[in] stream_id The stream identifier which has completed its HEADERS
-   * frame.
+   * @param[in] stream_id The stream identifier for which the end stream has
+   * been processed.
    */
-  void set_headers_are_available(int32_t stream_id);
+  void set_stream_has_ended(int32_t stream_id);
 
-  /// Whether headers have been received.
-  bool get_headers_are_available() const;
+  /// Whether an entire stream has been received and is ready for processing.
+  bool get_a_stream_has_ended() const;
 
   /// Return whether this session is for a listening server.
   bool get_is_server() const;
@@ -1914,8 +1920,7 @@ class H2Session : public TLSSession
   nghttp2_option *_options = nullptr;
   bool _h2_is_negotiated = false;
 
-  bool _headers_are_available = false;
-  std::deque<int32_t> _streams_with_headers;
+  std::deque<int32_t> _ended_streams;
   std::shared_ptr<H2StreamState> _last_added_stream;
 
 #ifndef OPENSSL_NO_NEXTPROTONEG