Allow persistent connections for HTTP PURGE (error) responses via 204

This reverts the revert in 7233f4b2fb966e681e37753d542882b03127c057,
  and changes the PURGE error codes from 200 to 204, which allows
  the vhtcpd parser to detect end-of-message correctly.

Change-Id: Ife5a0db654b46d467a886ec0834ae388439e8fdb