refactor: don't allow limit = 0

[yangby-cryptape]

Description

Personally, I feel 0 means no limit is a bit special. Without special treatment, it should mean forbidden.

Another way to change（for reference only）

storage.generate_spv_client_and_spv_update(spv_tip_height, None)? ;

  if let Some(limit) = limit {
      tip_height = std::cmp::min(tip_height, prev_height.saturating_add(limit));
  }

Originally posted by @EthanYuan in #14 (comment)