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/main.cpp

@@ -3893,6 +3893,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
             LOCK(pfrom->cs_filter);
             delete pfrom->pfilter;
             pfrom->pfilter = new CBloomFilter(filter);
+            filter.UpdateEmptyFull();
         }
         pfrom->fRelayTxes = true;
     }
@@ -3922,7 +3923,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
     {
         LOCK(pfrom->cs_filter);
         delete pfrom->pfilter;
-        pfrom->pfilter = NULL;
+        pfrom->pfilter = new CBloomFilter();
         pfrom->fRelayTxes = true;
     }