peer-cfg: Use an rwlock instead of a mutex to safely access child-cfgs

peer-cfg: Use an rwlock instead of a mutex to safely access child-cfgs

If multiple threads want to enumerate child-cfgs and potentially lock
other locks (e.g. check out IKE_SAs) while doing so a deadlock could
be caused (as was the case with VICI configs with start_action=start).
It should also improve performance for roadwarrior connections and lots
of clients connecting concurrently.

Fixes #2374.