Write and add linter to M3DB for detecting locks that are locked and then never unlocked

[richardartoul]

I.E the linter should be able to detect simple errors like this

func (s *someStruct) someMethod() error {
    s.Lock()
    err := s.doSomeWork()
    if err != nil {
        return err
    }
    s.Unlock()
    return nil
}

This will allow us to avoid defer's in hot-paths, but still be confident our code isn't buggy.

[prateek]

Did a little research on this a while back, more details here: m3db/build-tools#14