Prevent fingerprinting, disk-DoS with compact blocks

- Ignore GETBLOCKTXN requests for unknown blocks

Don't disconnect peers, or else we leak information that could be
used for fingerprinting.

- Ignore CMPCTBLOCK messages for pruned blocks

Also ignores CMPCTBLOCK announcements that have too little work.  This is to
prevent disk-exhaustion DoS.

Github-Pull: #8408
Rebased-From: 1de2a46632 1d06e49834

src/main.cpp

@@ -5344,7 +5344,6 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
         BlockMap::iterator it = mapBlockIndex.find(req.blockhash);
         if (it == mapBlockIndex.end() || !(it->second->nStatus & BLOCK_HAVE_DATA)) {
-            Misbehaving(pfrom->GetId(), 100);
             LogPrintf("Peer %d sent us a getblocktxn for a block we don't have", pfrom->id);
             return true;
         }
@@ -5628,8 +5627,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 std::vector<CInv> vInv(1);
                 vInv[0] = CInv(MSG_BLOCK, cmpctblock.header.GetHash());
                 pfrom->PushMessage(NetMsgType::GETDATA, vInv);
-                return true;
             }
+            return true;
         }
 
         // If we're not close to tip yet, give up and let parallel block fetch work its magic