Lock() of Locker interface should probably take a receive-only channel #175

Lock() of Locker interface should probably take a receive-only channel for stopChan, that is, instead of

Lock(stopChan chan struct{}) (<-chan struct{}, error)

have

Lock(<-stopChan chan struct{}) (<-chan struct{}, error)

Lock would not be writing to stopChan, the user of the Lock will - and this signature will allow using things like Context's Done channel directly (without mediating it with another select).