Use memmove rather than Array.Copy

[Buffer.BlockCopy](https://github.com/dotnet/coreclr/blob/bc146608854d1db9cdbcc0b08029a87754e12b49/src/vm/comutilnative.cpp#L1451)
is memmove; and a larger throughput intrinsic

[Array.Copy](https://github.com/dotnet/coreclr/blob/bc146608854d1db9cdbcc0b08029a87754e12b49/src/classlibnative/bcltype/arraynative.cpp#L895)
is 16 byte loop

Author: benaadams

src/Microsoft.AspNet.Server.Kestrel/Http/SocketOutput.cs

@@ -52,7 +52,7 @@ public Task WriteAsync(
             if (buffer.Array != null)
             {
                 var copy = new byte[buffer.Count];
-                Array.Copy(buffer.Array, buffer.Offset, copy, 0, buffer.Count);
+                Buffer.BlockCopy(buffer.Array, buffer.Offset, copy, 0, buffer.Count);
                 buffer = new ArraySegment<byte>(copy);
                 _log.ConnectionWrite(_connectionId, buffer.Count);
             }