OPTIM/MINOR: config: Optimize fullconn automatic computation loading configuration

OPTIM/MINOR: config: Optimize fullconn automatic computation loading configuration

The previous version used an O(number of proxies)^2 algo to get the sum of
the number of maxconns of frontends which reference a backend at least once.

This new version adds the frontend's maxconn number to the backend's
struct proxy member 'tot_fe_maxconn' when the backend name is resolved
for switching rules or default_backend statment.  At the end, the final
backend's fullconn is computed looping only one time for all on proxies O(n).

The load of a configuration using a large amount of backends (10 thousands)
without configured fullconn was reduced from several minutes to few seconds.