Feature request: Add option to get list of keys

[tfonfara]

Currently I have to know (by key) which cache entry I want to remove. My use case is that at some event multiple keys should get invalidated (e.g. all keys with prefix xy). Currently I can maintain my own list of keys using SetNewItemCallback and SetExpirationReasonCallback but as I would have to deal with concurrency there as well I would love to see something similar in this library!

Untested draft:

func (cache *Cache) GetKeys() ([]string, error) {
    cache.mutex.Lock()
    if cache.isShutDown {
        cache.mutex.Unlock()
        return nil, ErrClosed
    }

    keys := make([]string, len(cache.items))
    i := 0
    for k := range cache.items {
        keys[i] = k
        i++
    }

    cache.mutex.Unlock()

    return keys, nil
}

[tfonfara]

Added PR #43

[ReneKroon]

The MR is good, it's failing coverage but that's because of another small thing i merged on master recently. You can use master if you like. I'll clean up the build and release a full version within roughly a week.

[tfonfara]

Thanks, sounds good!