Use an IO thread pool executor by default

Use an IO thread pool executor by default

Summary:
Remove member variable name shadowing

Facebook: This diff adds a default executor for async requests which do not specify one. This ensures we don't run request callbacks on the event threads (which in turn ensures we don't make blocking tcc calls from the event threads)

Test Plan: fbconfig tao/client; fbmake runtests

Reviewed By: hannesr@fb.com

Subscribers: hannesr, fugalh, zhuohuang, folly-diffs@, tao-eng@

FB internal diff: D1731078

Tasks: 5752037

Signature: t1:1731078:1418237801:5b9bbe74c288292d5e7c7e6a38e67be864627d89