Compare and swap feature

[asppsa]

This is to add a feature that when supported by a store allows atomic swapping of a value, as suggested in #156. This method signature would be something like:

swap(key, old, new) => Boolean

returning true iff the swap happened. The most obvious name for the feature would be :swap.

[brutuscat]

@asppsa does this refers to CAS

[bjeanes]

@brutuscat yes