Summary:
The mcrouter fibers library is quite general purpose and reusable by other projects. Hence I'm moving it into folly.

Test Plan:
fbconfig -r folly/experimental/fibers && fbmake runtests
fbconfig -r mcrouter && fbmake runtests
fbconfig -r tao && fbmake
fbconfig -r ti && fbmake

Reviewed By: andrii@fb.com

Subscribers: vikas, zhuohuang, jmkaldor, jhunt, pavlo, int, aap, trunkagent, fredemmott, alikhtarov, folly-diffs@, jsedgwick, yfeldblum, chalfant, chip

FB internal diff: D1958061

Signature: t1:1958061:1428005194:b57bfecfe9678e81c48526f57e6197270e2b5a27