Fix McReply(ErrorReplyT) constructor
Fix McReply(ErrorReplyT) constructor

Summary: McReply(ErrorReplyT, folly::StringPiece) constructor creates IOBuf with CopyBufferOp, which will allocate SharedInfo. We don't need to even emplace IOBuf for empty string.
For each request we allocated such reply twice (RootRoute, DestinationRoute). Fixing this for previous OSS mcrouter release saved ~5% CPU (only DestinationRoute was calling this constructor).

Test Plan: tests, loadtest for previous OSS release.

Reviewed By: pavlo-fb

Differential Revision: D2548888

fb-gh-sync-id: b9450e3dac5960f18556c5770c3aa74ba4abc412

facebook/mcrouter,synecdoche/mcrouter,nvaller/mcrouter,yqzhang/mcrouter,facebook/mcrouter,yqzhang/mcrouter,synecdoche/mcrouter,facebook/mcrouter,nvaller/mcrouter,nvaller/mcrouter,nvaller/mcrouter,facebook/mcrouter,yqzhang/mcrouter,synecdoche/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter