MEDIUM: server: add and use srv_init() function

MEDIUM: server: add and use srv_init() function

rename _srv_postparse() internal function to srv_init() function and group
srv_init_per_thr() plus idle conns list init inside it. This way we can
perform some simplifications as srv_init() performs multiple server
init steps after parsing.

SRV_F_CHECKED flag was added, it is automatically set when srv_init()
runs successfully. If the flag is already set and srv_init() is called
again, nothing is done. This permis to manually call srv_init() earlier
than the default POST_CHECK hook when needed without risking to do things
twice.