HTTP2: Update local flow-controller on Channel.read() if needed (#9400)

Motivation:

We should better update the flow-controller on Channel.read() to reduce overhead and memory overhead.

See https://github.com/netty/netty/pull/9390#issuecomment-513008269

Modifications:

Move updateLocalWindowIfNeeded() to doBeginRead()

Result:

Reduce memory overhead

codec-http2/src/main/java/io/netty/handler/codec/http2/AbstractHttp2StreamChannel.java

@@ -751,6 +751,8 @@ abstract class AbstractHttp2StreamChannel extends DefaultAttributeMap implements
             if (!isActive()) {
                 return;
             }
+            updateLocalWindowIfNeeded();
+
             switch (readStatus) {
                 case IDLE:
                     readStatus = ReadStatus.IN_PROGRESS;
@@ -827,8 +829,6 @@ abstract class AbstractHttp2StreamChannel extends DefaultAttributeMap implements
                 readStatus = ReadStatus.IDLE;
             }
 
-            updateLocalWindowIfNeeded();
-
             allocHandle.readComplete();
             pipeline().fireChannelReadComplete();
             if (config().isAutoRead()) {