In cases where old unread messages in the home view might have been
leaked (either due to bugs or unusual muting interactions), it's
theoretically possible for the first unread message in the home view
to be far older than the pointer.
Since the Zulip mobile app is loading messages following the
use_first_unread logic, we need to plug this gap.
Probably a longer-term solution will involve changing how
update_message_flags works to automatically advance the pointer, but
this change should make it possible for the mobile apps to
consistently use the `use_first_unread` mechanism for fetching the
latest home view messages.
With tweaks to the tests by tabbott.
Fixeszulip/zulip-mobile#422.
This makes us more consistent, since we have other wrappers
like client_patch, client_put, and client_delete.
Wrapping also will facilitate instrumentation of our posting code.