MINOR: cfgparse/server: move (min/max)conn postparsing logic into dedicated function

MINOR: cfgparse/server: move (min/max)conn postparsing logic into dedicated function

In check_config_validity() function, we performed some consistency checks to
adjust minconn/maxconn attributes for each declared server.

We move this logic into a dedicated function named srv_minmax_conn_apply()
to be able to perform those checks later in the process life when needed
(ie: dynamic servers)