Make CRollingBloomFilter set nTweak for you

While CBloomFilter is usually used with an explicitly set nTweak,
CRollingBloomFilter is only used internally. Requiring every caller to
set nTweak is error-prone and redundant; better to have the class handle
that for you with a high-quality randomness source.

Additionally when clearing the filter it makes sense to change nTweak as
well to recover from a bad setting, e.g. due to insufficient randomness
at initialization, so the clear() method is replaced by a reset() method
that sets a new, random, nTweak value.

src/main.cpp

@@ -4812,7 +4812,7 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
             {
                 // Periodically clear addrKnown to allow refresh broadcasts
                 if (nLastRebroadcast)
-                    pnode->addrKnown.clear();
+                    pnode->addrKnown.reset();
 
                 // Rebroadcast our address
                 AdvertizeLocal(pnode);