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/net.cpp

@@ -2060,7 +2060,7 @@ unsigned int SendBufferSize() { return 1000*GetArg("-maxsendbuffer", 1*1000); }
 
 CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNameIn, bool fInboundIn) :
     ssSend(SER_NETWORK, INIT_PROTO_VERSION),
-    addrKnown(5000, 0.001, insecure_rand()),
+    addrKnown(5000, 0.001),
     setInventoryKnown(SendBufferSize() / 1000)
 {
     nServices = 0;