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/net.h

@@ -267,7 +267,7 @@ public:
         nMisbehavior = 0;
         fRelayTxes = false;
         setInventoryKnown.max_size(SendBufferSize() / 1000);
-        pfilter = NULL;
+        pfilter = new CBloomFilter();
 
         // Be shy and don't send version until we hear
         if (hSocket != INVALID_SOCKET && !fInbound)