Refactor MeshMS ply-reading functions

Rename functions to use _prev instead of _next to mean reading
"backwards" ie, from newest to oldest, aka, from higher offsets
to lower offsets
Improve MeshMS RESTful HTTP requests

Include local (my) and remote (their) SIDs in JSON responses for
conversationlist.json and messagelist.json, so that the same JSON
structures can be used in future for non-SID-specific queries.

Refactor MeshMS message iterator, rename "sender" and "recipient" fields
to "my" and "their", for consistency with the rest of the MeshMS source
code, and because "sender" and "recipient" are properties that apply to
a single message or single ply, not to a message thread (conversation).
Refactor rhizomehttp MeshMS message list test

New function add_messages() to use in fixture

Improve existing test case by adding more ACKs and messages
Implement HTTP GET /restful/meshms/<SID>/<SID>/newsince/<token>/messagelist.json

Two new test cases