Implement len caching for parking_lot RwLock (#10032)

- Refactor (and rename crate) and implement RwLock len cache.

- improve docs

- update ethcore verification queue to new version

- Implement Default, From,  also derive Debug

- update: parking_lot to 0.7

ethcore/src/verification/queue/mod.rs

@@ -29,7 +29,7 @@ use io::*;
 use error::{BlockError, ImportErrorKind, ErrorKind, Error};
 use engines::EthEngine;
 use client::ClientIoMessage;
-use len_caching_mutex::LenCachingMutex;
+use len_caching_lock::LenCachingMutex;
 
 use self::kind::{BlockLike, Kind};