Summary:
`folly::AtomicBitSet<N>` used `N` to denote the number of blocks
(32-bit or 64-bit each) instead of number of bits as used by
`std::bitset`. This confusion of unit led to unintentional allocation
of extra storage in many cases.

A simple fix would be to make `N` denote number of bits. However,
changing meaning of `N`could lead to silent breakage of code during
run time for some users who used `N` to denote number of blocks.

Therefore, to make the new meaning of `N` more explicit,
`AtomicBitSet` is renamed to `ConcurrentBitSet` & `N` now denotes the
number of bits.

Reviewed By: al13n321

Differential Revision: D18866751

fbshipit-source-id: 17741646862776e1a080eaac7877f495bb56b656