Rename shadowing local variables in various stats objects

Rename shadowing local variables in various stats objects

Summary:
The GCC shadow variable detector is quite zealous in warning about
shadowed members. Here, we have local variables that share the name
of member functions. Rename them so that -Wshadow doesn't complain.

Test Plan:
It didn't build, and now it does.

`fbconfig -r folly && fbmake runtests`

Reviewed By: njormrod@fb.com

Subscribers: fbcode-common-diffs@, njormrod

FB internal diff: D1534033