Do the set up of RTP receivers synchronously so that they
are ready to receive media as soon as signalling to remote
side is sent. Once signalling to remote side is sent, the
remote can send media at any time and receiver has to be ready.

Like the bug mentions, a negotiation quickly followed by
a renegotiation left the RTP receivers of the tracks in the
second offer not set up. If the tracks in the renegotiation
happen to be simulcast tracks, they are missed as browsers
send RID only in the first few packets.

The problem can be reproduced by introducing a 1 second
delay in Downstream direction in Network Link Conditioner
and using a modified version of LiveKit JS SDK sample app to
force a double negotiation spaced closely.

With this change, onTrack fires, but the unhandled warning
from RTCP for the highest layer still happens. But, the
track fires almost immediately following that warning
(less than 5 ms later). So, all the simulcast layers
are available.

Resolves #2054