Need to call send_headers explicitly
Add object file names
Add response finish function
Remove useless variables
Allow appending data to existing writing action
on_write_finished is also registered for writing headers
Revert "Add response finish function"

No need to use response_finish, should use handler return values instead

This reverts commit 4fcccd529b83792ee66e0823fc2ba2ffe4ea6492.
Add _done flag to indicate whether a response is finished or not