[MEDIUM] backend: implement consistent hashing variation

[MEDIUM] backend: implement consistent hashing variation

Consistent hashing provides some interesting advantages over common
hashing. It avoids full redistribution in case of a server failure,
or when expanding the farm. This has a cost however, the hashing is
far from being perfect, as we associate a server to a request by
searching the server with the closest key in a tree. Since servers
appear multiple times based on their weights, it is recommended to
use weights larger than approximately 10-20 in order to smoothen
the distribution a bit.

In some cases, playing with weights will be the only solution to
make a server appear more often and increase chances of being picked,
so stats are very important with consistent hashing.

In order to indicate the type of hashing, use :

   hash-type map-based      (default, old one)
   hash-type consistent     (new one)

Consistent hashing can make sense in a cache farm, in order not
to redistribute everyone when a cache changes state. It could also
probably be used for long sessions such as terminal sessions, though
that has not be attempted yet.

More details on this method of hashing here :
  http://www.spiteful.com/2008/03/17/programmers-toolbox-part-3-consistent-hashing/