Rename CursorResponse to PostCursorResponse for consistency

[DiscoPYF]

CursorResponse doesn't follow the naming we use for all the responses. Renaming it to PostCursorResponse will be more consistent with how other responses are named. I find it is helpful to rename it because my first thought is to declare PostCursorResponse for the return type when calling PostCursorAsync.

This may be a breaking change depending on how the change is implemented, but it would be better to do it before we release v1.

[DiscoPYF]

Better to wait for #326 after all.

[DiscoPYF]

This issue was reverted temporarily in #433