Retire the node-local interface (port 5986)

[wohali]

This can be done in two phases:

1. Move relevant 5986 port functions under /_node/<nodename>/... such as:
   • _dbs
   • _nodes
   • per-shard DB operations (all)
2. Actually eliminate couch_httpd.

@davisp thought this might go under the new HTTP/2 API/internals but I think they're distinct efforts.

[denib]

@rnewson took over the WIP PR: #2092
This is in progress.