Add atomic "Get and Set" method

[padjoo]

• Adds an atomic Get plus Set method with a decider function to allow client to lookup value and decide on the new value (or leave as is).
• This would allow use cases like Incr, Decr as well as more complicated flows where setting the value depends on the current value.

[coocood]

Thank you for the PR!
It's a very useful feature
How about rename it Update for the method and Updater for the function?

[padjoo]

Thanks @coocood!