python-memcached misses working backend

In multi-backend case there is always a chance for client to miss a working backend among not responding.

Assuming memcached is up and running on localhost:11211:

import memcache
import random
cl = memcache.Client(['localhost:11211', 'localhost:11212', 'localhost:11213', 'localhost:11214'])
while cl.set(str(random.random()), 1): pass

'while' must be an infinite loop since cl.set() returns True on success but it exits in milliseconds instead.

Bug may be fixed by changing backend selection logic, patch attached.

The other way is to use https://github.com/linsomniac/python-memcached2/ as linsomniac suggested: https://github.com/linsomniac/python-memcached/pull/57