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/bloom.h

@@ -89,6 +89,7 @@ public:
     bool contains(const uint256& hash) const;
 
     void clear();
+    void reset(unsigned int nNewTweak);
 
     //! True if the size is <= MAX_BLOOM_FILTER_SIZE and the number of hash functions is <= MAX_HASH_FUNCS
     //! (catch a filter which was just deserialized which was too big)
@@ -103,7 +104,11 @@ public:
 
 /**
  * RollingBloomFilter is a probabilistic "keep track of most recently inserted" set.
- * Construct it with the number of items to keep track of, and a false-positive rate.
+ * Construct it with the number of items to keep track of, and a false-positive
+ * rate. Unlike CBloomFilter, by default nTweak is set to a cryptographically
+ * secure random value for you. Similarly rather than clear() the method
+ * reset() is provided, which also changes nTweak to decrease the impact of
+ * false-positives.
  *
  * contains(item) will always return true if item was one of the last N things
  * insert()'ed ... but may also return true for items that were not inserted.
@@ -111,14 +116,15 @@ public:
 class CRollingBloomFilter
 {
 public:
-    CRollingBloomFilter(unsigned int nElements, double nFPRate, unsigned int nTweak);
+    CRollingBloomFilter(unsigned int nElements, double nFPRate,
+                        unsigned int nTweak = 0);
 
     void insert(const std::vector<unsigned char>& vKey);
     void insert(const uint256& hash);
     bool contains(const std::vector<unsigned char>& vKey) const;
     bool contains(const uint256& hash) const;
 
-    void clear();
+    void reset(unsigned int nNewTweak = 0);
 
 private:
     unsigned int nBloomSize;