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
UNFINISHED Implement HTTP GET /restful/meshms/<SID>/<SID>/newsince/<token>/messagelist.json