Data frames release the underlying ByteBuf, not the wrapped Buf (link…

…erd#1751)

The Finagle `ByteBufAsBuf` converter wraps a Netty 4 `ByteBuf`, but does not support reference counting (see [this comment](https://github.com/twitter/finagle/blob/82d00660373582d6bfe56df8155b52528df36691/finagle-netty4/src/main/scala/com/twitter/finagle/netty4/ByteBufAsBuf.scala#L58-L61)). Therefore, when releasing a data frame whose content `ByteBuf` we `retain()` before wrapping it as a `Buf`, we must `release()` the underlying `ByteBuf`. 

This fixes the memory leak observed in linkerd#1678 without removing the call to `retain()` which was removed in linkerd#1711, which introduced a data corruption issue (linkerd#1728). If we `release()` the underlying `ByteBuf` after releasing the data frame, we can once again `retain()` it before converting to `Buf` without leaking, thus avoiding the data corruption caused by removing the `retain()`.

Thanks to @vadimi for helping with this fix!

Fixes linkerd#1728.

finagle/h2/src/main/scala/com/twitter/finagle/buoyant/h2/netty4/Netty4Message.scala

@@ -102,10 +102,19 @@ private[h2] object Netty4Message {
 
     def apply(f: Http2DataFrame, updateWindow: Int => Future[Unit]): Frame.Data = {
       val sz = f.content.readableBytes + f.padding
-      val buf = ByteBufAsBuf(f.content)
+      val buf = ByteBufAsBuf(f.content.retain())
       val releaser: () => Future[Unit] =
-        if (sz > 0) () => updateWindow(sz)
-        else () => Future.Unit
+        () =>
+          {
+            // When releasing the frame, call `updateWindow` with the frame's
+            // size to release the frame's window capacity.
+            val res = if (sz > 0) updateWindow(sz) else Future.Unit
+            // Because `ByteBufAsBuf` does not support reference counting,
+            // it's necessary to call `.release()` on the underlying `ByteBuf`
+            // when releasing the data frame.
+            f.content.release()
+            res
+          }
       Frame.Data(buf, f.isEndStream, releaser)
     }
   }