httpserver: use own HTTP status codes
httpserver: use own HTTP status codes

Before, macros defined in `<event2/http.h>` have been used
for some HTTP status codes.
`<event2/http.h>` is included implicitly and the usage
of its status code macros is inconsistent with the majority
HTTP response implementations in this file.

Now, the `HTTPStatusCode` enum from `<rpc/protocol.h>` is
consistently used for all HTTP response implementations.

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