Change NAT check to increase time between checks and prevent proxies from reverting to unknown

Something went wrong with how we are doing the probetest and our counts of unrestricted proxies dropped from 77 to almost zero overnight. I don't know the cause, but I'd like to propose the following change in how we handle NAT checks:

• proxies will only perform the check on startup or once a week
• unrestricted proxies that have too many failures will trigger a check
• if a proxy had a concrete NAT type (restricted or unrestricted) before the check, and the check comes back inconclusive (unknown), keep the old NAT type.