[MAJOR] implemented the 'minconn' server parameter for dynamic load regulation

[MAJOR] implemented the 'minconn' server parameter for dynamic load regulation
When 'minconn' is set, the number of simultaneous sessions sent to the server
will be limited by a dynamic value depending on the global load on the
instance itself. The principle is to fix the maximal concurrency on the server
proportionnally to the instance's usage relative to its maxconn, with a minimum
fixed to <minconn>. The formula for the number of simultaneous sessions sent
to the server is then max(srv_minconn, srv_maxconn*px_conn/px_maxconn). This
helps unloading the servers when the load is very low.