CAddrMan: stochastic address manager

Design goals:
 * Only keep a limited number of addresses around, so that addr.dat does not grow without bound.
 * Keep the address tables in-memory, and occasionally write the table to addr.dat.
 * Make sure no (localized) attacker can fill the entire table with his nodes/addresses.

See comments in addrman.h for more detailed information.

src/uint256.h

@@ -348,6 +348,10 @@ public:
         return sizeof(pn);
     }
 
+    uint64 Get64(int n=0) const
+    {
+        return pn[2*n] | (uint64)pn[2*n+1] << 32;
+    }
 
     unsigned int GetSerializeSize(int nType=0, int nVersion=PROTOCOL_VERSION) const
     {