Don't expose NetLocalHandle or make it !Send

[wangrunji0408]

NetLocalHandle is the entry point of all network operations. However, it should not be sent to other threads. One thread using a handle from other thread would cause it to block forever. The current API design is likely to be misused. For example, users tend to store the handle in a global state. To prevent that, we should either hide this type and make its methods static, or make this type !Send.

[wangrunji0408]

Removed in f014a65