[MAJOR] implementation of the "leastconn" load balancing algorithm

[MAJOR] implementation of the "leastconn" load balancing algorithm

The new "leastconn" LB algorithm selects the server which has the
least established or pending connections. The weights are considered,
so that a server with a weight of 20 will get twice as many connections
as the server with a weight of 10.

The algorithm respects the minconn/maxconn settings, as well as the
slowstart since it is a dynamic algorithm. It also correctly supports
backup servers (one and all).

It is generally suited for protocols with long sessions (such as remote
terminals and databases), as it will ensure that upon restart, a server
with no connection will take all new ones until its load is balanced
with others.

A test configuration has been added in order to ease regression testing.