Fixed server enumeration logic

Created with a list of memcache servers client has non-zero chance
to fail any operation involving server selection due to _get_server()
sometimes fails to find single alive server among dead ones.

Changed server search routine to perform each new server search
excluding dead server.

Updated to run under Python 3

 Bug filed in Mirantis OpenStack:
 https://bugs.launchpad.net/mos/+bug/1370324