implementing message-handling forthe new protocol v2

    - API changes: all the _async() functions have been droppedin favour of
      a more generic API which defaults to asynchronous operations and behaves
      synchronously only if no callback has been passed (only exception for the
      get and offset operations for which instead an explicit synchronous interface
      has been provided to not make the default interface too complex.
    - implemented the CAS command
    - implemented the *_MULTI commands

(WIP)