Remove "externals"

[rnewson]

Overview

Remove all the plumbing that enabled _external/ request handling,
leaving only the functions necessary for list and show.

closes #2166

Testing recommendations

N/A

Related Issues or Pull Requests

N/A

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation

[kocolosk]

This is nicely conservative. One could imagine a more substantive refactor to remove the chttpd_external and couch_httpd_external modules entirely, but that should wait.

[rnewson]

I think once _show and _list are removed, we can remove those modules but I didn't think now was the right time to move these (ancient) functions or attempt to consolidate them (there's a ton of overlap but perhaps subtle differences). Will merge if the Travis fairy grants my wish.