Replace mruset setAddrKnown with CRollingBloomFilter addrKnown

Use a probabilistic bloom filter to keep track of which addresses
we think we have given our peers, instead of a list.

This uses much less memory, at the cost of sometimes failing to
relay an address to a peer-- worst case if the bloom filter happens
to be as full as it gets, 1-in-1,000.

Measured memory usage of a full mruset setAddrKnown: 650Kbytes
Constant memory usage of CRollingBloomFilter addrKnown: 37Kbytes.

This will also help heap fragmentation, because the 37K of storage
is allocated when a CNode is created (when a connection to a peer
is established) and then there is no per-item-remembered memory
allocation.

I plan on testing by restarting a full node with an empty peers.dat,
running a while with -debug=addrman and -debug=net, and making sure
that the 'addr' message traffic out is reasonable.
(suggestions for better tests welcome)

src/net.h

@@ -300,7 +300,7 @@ public:
 
     // flood relay
     std::vector<CAddress> vAddrToSend;
-    mruset<CAddress> setAddrKnown;
+    CRollingBloomFilter addrKnown;
     bool fGetAddr;
     std::set<uint256> setKnown;
 
@@ -380,7 +380,7 @@ public:
 
     void AddAddressKnown(const CAddress& addr)
     {
-        setAddrKnown.insert(addr);
+        addrKnown.insert(addr.GetKey());
     }
 
     void PushAddress(const CAddress& addr)
@@ -388,7 +388,7 @@ public:
         // Known checking here is only to save space from duplicates.
         // SendMessages will filter it again for knowns that were added
         // after addresses were pushed.
-        if (addr.IsValid() && !setAddrKnown.count(addr)) {
+        if (addr.IsValid() && !addrKnown.contains(addr.GetKey())) {
             if (vAddrToSend.size() >= MAX_ADDR_TO_SEND) {
                 vAddrToSend[insecure_rand() % vAddrToSend.size()] = addr;
             } else {