[finagle-core] Don't make an unnecessary copy of the array with frames in LengthFieldFramer

Problem:
We unnecessarily copy the array with extracted frames into an indexed seq.

Solution:
Don't copy

Differential Revision: https://phabricator.twitter.biz/D1164516

Author: mbezoyan

finagle-core/src/main/scala/com/twitter/finagle/decoder/LengthFieldFramer.scala

@@ -1,6 +1,7 @@
 package com.twitter.finagle.decoder
 
-import com.twitter.io.{Buf, ByteReader}
+import com.twitter.io.Buf
+import com.twitter.io.ByteReader
 import scala.collection.mutable.ArrayBuffer
 
 private[twitter] object LengthFieldFramer {
@@ -150,7 +151,7 @@ private[twitter] class LengthFieldFramer(
         }
 
         accum = accum.slice(frameCursor, accum.length)
-        frames.toIndexedSeq
+        frames.asInstanceOf[IndexedSeq[Buf]]
       } else {
         NoFrames
       }