The commit message of "Associate remote participant w/ JitsiConference
(_JOINED)" explains the motivation for this commit.

Practically, _JOINED and _LEFT combined with "Remove remote participants
who are no longer of interest" should alleviate the problem with
multiplying remote participants to an acceptable level of annoyance.

Technically though, a remote participant cannot be identified by an ID
only. The ID is (somewhat) "unique" in the context of a single
JitsiConference instance. So in order to not have to scratch our heads
over an obscure corner, racing case, it's better to always identify
remote participants by the pair id-conference. Unfortunately, that's a
bit of a high order given the existing source code. So I've implemented
the cases which are the easiest so that new source code written with
participantUpdated is more likely to identify a remote participant with
the pair id-conference.

Additionally, the commit "Reduce direct read access to the
features/base/participants redux state" brings more control back to the
functions of the feature base/participants so that one day we can (if we
choose to) do something like, for example:

If getParticipants is called with a conference, it returns the
participants from features/base/participants who are associated with the
specified conference. If no conference is specified in the function
call, then default to the conference which is the primary focus of the
app at the time of the function call. Added to the above, this should
allow us to further reduce the cases in which we're identifying remote
participants by id only and get us even closer to a more "predictable"
behavior in corner, racing cases.