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/protocol.h

@@ -94,7 +94,7 @@ class CAddress : public CService
         unsigned int nTime;
 
         // memory only
-        unsigned int nLastTry;
+        int64 nLastTry;
 };
 
 class CInv