Fix race condition that could cause pairing state to be lost

If a mDNS response for a host happened during the period where we
had removed the host from discovery, it would be re-added by mDNS
without the pairing cert. Now we no longer completely remove the host
and instead just remember that it's paused.

Limelight/Network/DiscoveryManager.h

@@ -24,6 +24,8 @@
 - (void) resetDiscoveryState;
 - (BOOL) addHostToDiscovery:(TemporaryHost*)host;
 - (void) removeHostFromDiscovery:(TemporaryHost*)host;
+- (void) pauseDiscoveryForHost:(TemporaryHost *)host;
+- (void) resumeDiscoveryForHost:(TemporaryHost *)host;
 - (void) discoverHost:(NSString*)hostAddress withCallback:(void (^)(TemporaryHost*, NSString*))callback;
 
 @end