Fix some clang compiler warnings/errors

Fix some clang compiler warnings/errors

Summary:
Fixes instances of the following problems:
error: offset of on non-POD type 'folly::fbstring_core<char>::RefCounted'
[-Werror,-Winvalid-offsetof]
Solution: Since the atomic is not a POD I used sizeof() instead to calculate
the offset

warning: C++11 requires lambda with omitted result type to consist of a single
return statement
Solution: Specify a return type

error: in-class initializer for static data member is not a constant expression
Solution: Move initializer

Test Plan: - Compiled folly and ran fbmake runtests

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D656963