Implement balancing strategy in backend choice

[hamadodene]

At the moment the choice of the backend is done randomly by shuffling the list of backends.
You might consider implementing a balancing strategy in choosing backend such as:

• Round-robin
• Least Connections --> Send the request to the backend with the least number of active connections
• Weighted Round robin

It might be interesting to give the possibility to choose which balance to use.
Something like:

director.1.balancingstrategy= round-robin|weighted|least-connection|random