http2: multiple style and performance updates

* move CHECK statements into DEBUG checks
* improve performance by removing branches
  * Several if checks were left in while the code was being developed.
    Now that the core API has stablized more, the checks are largely
    unnecessary and can be removed, yielding a significant boost in
    performance.
* refactor flow control for proper backpressure
* use std::queue for inbound headers
* use std::queue for outbound data
* remove now unnecessary FreeHeaders function
* expand comments and miscellaneous edits
* add a couple of misbehaving flow control tests

PR-URL: nodejs/node#16239
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/http2/core.js

@@ -279,19 +279,13 @@ function onSessionRead(nread, buf, handle) {
   assert(stream !== undefined,
          'Internal HTTP/2 Failure. Stream does not exist. Please ' +
          'report this as a bug in Node.js');
-  const state = stream[kState];
   _unrefActive(owner); // Reset the session timeout timer
   _unrefActive(stream); // Reset the stream timeout timer
+  if (nread >= 0 && !stream.destroyed)
+    return stream.push(buf);
 
-  if (nread >= 0 && !stream.destroyed) {
-    if (!stream.push(buf)) {
-      this.streamReadStop(id);
-      state.reading = false;
-    }
-  } else {
-    // Last chunk was received. End the readable side.
-    stream.push(null);
-  }
+  // Last chunk was received. End the readable side.
+  stream.push(null);
 }
 
 // Called when the remote peer settings have been updated.
@@ -1205,9 +1199,7 @@ function streamOnResume() {
     return;
   }
   const session = this[kSession];
-  const state = this[kState];
-  if (session && !state.reading) {
-    state.reading = true;
+  if (session) {
     assert(session[kHandle].streamReadStart(this[kID]) === undefined,
            `HTTP/2 Stream ${this[kID]} does not exist. Please report this as ` +
            'a bug in Node.js');
@@ -1216,9 +1208,7 @@ function streamOnResume() {
 
 function streamOnPause() {
   const session = this[kSession];
-  const state = this[kState];
-  if (session && state.reading) {
-    state.reading = false;
+  if (session) {
     assert(session[kHandle].streamReadStop(this[kID]) === undefined,
            `HTTP/2 Stream ${this[kID]} does not exist. Please report this as ' +
            'a bug in Node.js`);
@@ -1412,12 +1402,8 @@ class Http2Stream extends Duplex {
       return;
     }
     _unrefActive(this);
-    const state = this[kState];
-    if (state.reading)
-      return;
-    state.reading = true;
-    assert(this[kSession][kHandle].streamReadStart(this[kID]) === undefined,
-           `HTTP/2 Stream ${this[kID]} does not exist. Please report this as ` +
+    assert(this[kSession][kHandle].flushData(this[kID]) === undefined,
+           'HTTP/2 Stream #{this[kID]} does not exist. Please report this as ' +
            'a bug in Node.js');
   }