Add missing cache client methods

[pgautier404]

The following cache client operations are unimplemented:

• keysExist
• increment
• listConcatenateFront
• listConcatenateBack
• setAddElements
• setRemoveElements
• setContainsElements
• setContainsElement

[pgautier404]

keysExist is added in #99. Holding off on implementing the rest as they aren't 1.0 blockers.