[NatUtility] Fix a race condition with the search bool
[NatUtility] Fix a race condition with the search bool

Replace the 'search' bool with a WaitHandle so that SearchAndListen does
not have to poll constantly when searching is disabled.

Signed-off-by: Alan McGovern <49c93f7b16225e84d0e5a3b44c29062440c3ef45@gmail.com>

nterry/Mono.Nat,nterry/Mono.Nat,OpenRA/Mono.Nat,lontivero/Open.NAT,mono/Mono.Nat,OpenRA/Mono.Nat,lovewitty/Open.NAT,mcatanzariti/Open.NAT