Factored out multicast propagation algorithm from Switch and Topology, also cleaned up and clarified it a bit.

node/Constants.hpp

@@ -242,6 +242,11 @@ error_no_ZT_ARCH_defined;
  */
 #define ZT_MULTICAST_DEDUP_HISTORY_EXPIRE 8000
 
+/**
+ * Number of bits to randomly "decay" in bloom filter per hop
+ */
+#define ZT_MULTICAST_BLOOM_FILTER_DECAY_RATE 2
+
 /**
  * Period between announcements of all multicast 'likes' in ms
  *