modify Delete method to return deleted value

[matthyx]

This is useful for SafeMap where you cannot Lock() to perform Get() and Delete() atomically.

[spekary]

I think this is a great idea. Looking through the code, I don't see anything wrong with changing the interface for Delete to also return the value. That shouldn't break any current code, and it just adds a map lookup to Delete in all the versions, so it shouldn't significantly impact performance. Do you think you could change your code to just have Delete return the value, instead of adding another function?

[matthyx]

Sure I'll update it tomorrow!

[matthyx]

@spekary done!