rename caffe2::mobile_threadpool to caffe2::mobile_pthreadpool
Summary:
Rename old mobile_threadpool() API, replace it with a new version that
returns caffe2::ThreadPool instead of pthreadpool_t.
Test Plan: - builds
Differential Revision: D17543413
Pulled By: ljk53
fbshipit-source-id: a3effd24e8ce9d677a2a04ebe6b6e1582e6f0a65
