This commit follows up b6c5a2da08
by serializing the user's thread memberships in these cases:
1. When we do the initial channel fetch with messages, we get
all threads and all the user's thread memberships for those
messages.
2. When the thread list is fetched, we get all the user's memberships
in that list.
3. When the single thread is fetched, either from opening it from
the list, an OM indicator, or just from doing .find() on the
manager when a new MessageBus message comes in
This will let us track the lastReadMessageId on the client, and
will also let us fix an issue where the unread indicator in the
channel header was incrementing for every thread that got a
new message, regardless of whether the user was a member.
c3779a371f