Add method SetCb

[1xxz188]

The purpose is to support some atomic operations

[cristaloleg]

Can you clarify the use case? Why it cannot be just:

err := cache.Set("key", []byte("value"))
callback()

?

[1xxz188]

in order to verify that it is not affected when cb was nil.

[1xxz188]

Long story, wait until I have time to reply.