Denial-of-service flood control

Drop connections that are either sending messages too fast to handle or are processing messages so slowly data starts to back up.
Adds two new options:
  -maxreceivebuffer  Default: 2000 (2000*1000 bytes)
  -maxsendbuffer     Default: 256  (256*1000 bytes)

net.cpp

@@ -748,6 +748,12 @@ void ThreadSocketHandler2(void* parg)
                     CDataStream& vRecv = pnode->vRecv;
                     unsigned int nPos = vRecv.size();
 
+                    if (nPos > 1000*GetArg("-maxreceivebuffer", 2*1000)) {
+                        if (!pnode->fDisconnect)
+                            printf("socket recv flood control disconnect (%d bytes)\n", vRecv.size());
+                        pnode->CloseSocketDisconnect();
+                    }
+                    else {
                         // typical socket buffer is 8K-64K
                         char pchBuf[0x10000];
                         int nBytes = recv(pnode->hSocket, pchBuf, sizeof(pchBuf), MSG_DONTWAIT);
@@ -777,6 +783,7 @@ void ThreadSocketHandler2(void* parg)
                         }
                     }
                 }
+            }
 
             //
             // Send
@@ -806,6 +813,11 @@ void ThreadSocketHandler2(void* parg)
                                 pnode->CloseSocketDisconnect();
                             }
                         }
+                        if (vSend.size() > 1000*GetArg("-maxsendbuffer", 256)) {
+                            if (!pnode->fDisconnect)
+                                printf("socket send flood control disconnect (%d bytes)\n", vSend.size());
+                            pnode->CloseSocketDisconnect();
+                        }
                     }
                 }
             }