MINOR: server: group postinit server tasks under _srv_postparse()

MINOR: server: group postinit server tasks under _srv_postparse()

init_srv_requeue() and init_srv_slowstart() functions are called after
initial server parsing via REGISTER_POST_SERVER_CHECK() hook, and they
are also manually called for dynamic server after the server is
initialized.

This may conflict with _srv_postparse() which is also registered via
REGISTER_POST_SERVER_CHECK() and called during dynamic server creation

To ensure functions don't conflict with each other, let's ensure they
are executed in proper order by calling init_srv_requeue and
init_srv_slowstart() from _srv_postparse() which now becomes the parent
function for server related postparsing stuff. No change of behavior is
expected.