No need for cursorId to ever be null?

[kickscondor]

This is a change I made to fix a bug I was seeing when using hxcpp to compile this mongodb driver. The error was something like "no 'get' method on the 'Dynamic' cursorId" in Cursor.hx. I'm unfamiliar with hxcpp's source, so I looked into the Cursor.hx and couldn't see a reason why cursorId was typed Null<Int64> since Protocol::response returns only Int64.

I'll work on sorting out a fix for hxcpp as well. Huge thanks for this project!!