reactor: update to parking_lot 0.7

[dekellum]

Updates reactor parking_lot crate dependency to 0.7.0. This latest version of parking_lot (and parking_lot_core, lock_api) has minimum rustc 1.24, which is less than tokio's current 1.26, so no problem there.

cc: @Amanieu

Motivation

I've been waiting/watching for rand* crate duplicates and shim dependencies to go away, and this new parking_lot update removes the last old rand dep, which looked like this (via cargo tree -d):

 
rand_core v0.2.2
└── rand v0.5.5
    └── parking_lot_core v0.3.1
        └── parking_lot v0.6.4
            └── tokio-reactor v0.1.7 

[tobz]

@dekellum Is there anything major in the diff between 0.6.3 and 0.7.0 that you think is worth looking at? Tests pass, which is good, but I know that we've had recent issues with performance regressions. (Granted, this isn't surgery on the reactor/threadpool but might influence performance all the same.)

[Amanieu]

There was a small API change to Condvar: notify_one and notify_all now return the number of threads woken up.

There should be no difference in performance.