[MINOR] config: automatically compute a default fullconn value

[MINOR] config: automatically compute a default fullconn value

The fullconn value is not easy to get right when doing dynamic regulation,
as it should depend on the maxconns of the frontends that can reach a
backend. Since the parameter is mandatory, many configs are found with
an inappropriate default value.

Instead of rejecting configs without a fullconn value, we now set it to
10% of the sum of the configured maxconns of all the frontends which are
susceptible to branch to the backend. That way if new frontends are added,
the backend's fullconn automatically adjusts itself.