New util LenCachingMutex can be used in place of a Mutex, when working
with collections, or other types with a len() method. When
the Guard is dropped, the value returned from len() is stored
into an AtomicUsize and can be queried using load_len() without
needing to lock the Mutex. Implementations for ```Vec``` and
```VecDeque```.
Now used in [Verification](https://github.com/paritytech/parity-ethereum/blob/4ded4181a651fe9c26850f908e4ab63bf952cab6/ethcore/src/verification/queue/mod.rs#L196) so that calls to ```VerificationQueue.queue_info()```
no longer require locks.
mattrutherford
ec886ddefb