Introduce isBusyError for memcache
Introduce isBusyError for memcache

Summary: return mc_res_busy when mcrouter is overloaded. Based on this proxygen may implement load shedding for their clients. There is still a very valid point - it's super hard to get our queue limits right. usadiq has ideas on how to improve admission control in mcrouter/ratelim.

Differential Revision: D3116106

fb-gh-sync-id: 0e17ee8ce8fbdd557d706efb3fec24d9212e10c1
fbshipit-source-id: 0e17ee8ce8fbdd557d706efb3fec24d9212e10c1

yqzhang/mcrouter,yqzhang/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,yqzhang/mcrouter,yqzhang/mcrouter,facebook/mcrouter