Makes sure that whenever a conference is left or switched, the local
participant's id will be equal to the default value.

The problem fixed by this commit is a situation where the local
participant may end up sharing the same ID with it's "ghost" when
rejoining a disconnected conference. The most important and easiest to
hit case is when the conference is left after the CONFERENCE_FAILED
event.

Another rare and harder to encounter in the real world issue is
where CONFERENCE_LEFT may come with the delay due to it's asynchronous
nature. The step by step scenario is as follows: trying to leave a
conference, but the network is not doing well, so it takes time,
requests are timing out. After getting back to the welcome page the
the CONFERENCE_LEFT has not arrived yet. The same conference is joined
again and the load config may timeout, but it will be read from the
cache. Now the network gets better and conference is joining which
results in our ghost participant added to the redux state. At this point
there's the root issue: two participants with the same id, because the
local one was neither cleared nor set to the new one yet
(PARTICIPANT_JOINED come, before CONFERENCE_JOINED where we adjust the
id). Then comes CONFERENCE_JOINED and we try to update our local id.
We're updating the ID of both ghost and local participant. It could be
also that the delayed CONFERENCE_LEFT comes for the old conference, but
it's too late and it would update the id for both participants.

The approach here reasons that the ID of the local participant
may be reset as soon as the local participant and, respectively, her ID
is no longer involved in a recoverable JitsiConference of interest to
the user and, consequently, the app.

Co-authored-by: Pawel Domas <pawel.domas@jitsi.org>
Co-authored-by: Lyubo Marinov <lmarinov@atlassian.com>