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/lib.rs

@@ -112,7 +112,7 @@ extern crate journaldb;
 extern crate serde;
 #[cfg(any(test, feature = "json-tests", feature = "test-helpers"))]
 extern crate tempdir;
-extern crate len_caching_mutex;
+extern crate len_caching_lock;
 
 #[cfg(any(target_os = "linux", target_os = "macos", target_os = "windows", target_os = "android"))]
 extern crate hardware_wallet;

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};