cryptonote_protocol: drop peers we can't download from when syncing

Any peer that's behind us while syncing is useless to us (though
not to them). This ensures that we don't get our peer slots filled
with peers that we can't use. Once we've synced, we can connect
to them and they can then sync off us if they want.

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -341,6 +341,11 @@ namespace cryptonote
 
     if(m_core.have_block(hshd.top_id))
     {
+      if (target > hshd.current_height)
+      {
+        MINFO(context << "peer is not ahead of us and we're syncing, disconnecting");
+        return false;
+      }
       context.m_state = cryptonote_connection_context::state_normal;
       if(is_inital && target == m_core.get_current_blockchain_height())
         on_connection_synchronized();