Performance optimization for bloom filters.

This reduces a peer's ability to attack network resources by
 using a full bloom filter, but without reducing the usability
 of bloom filters.  It sets a default match everything filter
 for peers and it generalizes a prior optimization to
 cover more cases.

src/bloom.h

@@ -42,6 +42,8 @@ class CBloomFilter
 {
 private:
     std::vector<unsigned char> vData;
+    bool isFull;
+    bool isEmpty;
     unsigned int nHashFuncs;
     unsigned int nTweak;
     unsigned char nFlags;
@@ -57,9 +59,7 @@ public:
     // It should generally always be a random value (and is largely only exposed for unit testing)
     // nFlags should be one of the BLOOM_UPDATE_* enums (not _MASK)
     CBloomFilter(unsigned int nElements, double nFPRate, unsigned int nTweak, unsigned char nFlagsIn);
-    // Using a filter initialized with this results in undefined behavior
-    // Should only be used for deserialization
-    CBloomFilter() {}
+    CBloomFilter() : isFull(true) {}
 
     IMPLEMENT_SERIALIZE
     (
@@ -83,6 +83,9 @@ public:
 
     // Also adds any outputs which match the filter to the filter (to match their spending txes)
     bool IsRelevantAndUpdate(const CTransaction& tx, const uint256& hash);
+
+    // Checks for empty and full filters to avoid wasting cpu
+    void UpdateEmptyFull();
 };
 
 #endif /* BITCOIN_BLOOM_H */