read_receipt: add logic when alone in a conversation

No logic was implemented when you were alone in a conversation
(either conv with yourself or swarm with 1 member). This adds
the logic. Currently the receipts will change in swarms
if you add a member.

GitLab: #1867
GitLab: #1017
Change-Id: Iff2ea5978ac988d30edef32d803c0b5b70b11d90