Use Round Robin algorithm instead of random distribution

[merlos]

Current implementation requests tiles in a uniform random way. Each tile server has the same probability of being requested, however because of the randomness this can make that a server is hit 5 times in a row whereas the others are idle.

In order to improve this some kind of round robin strategy can be applied. We assume all servers are equally efficient and the requests are divided in sequence. With round robin the requests for a 3 subdomain server (a,b,c) the sequence of requests would be to the servers a, b, c, a, b, c, a, b, c, a, b, c, a, b, c...