* Revert "Resolve `join` promise after API channel ready (#185)" (#190)

This reverts commit 4d1d436.

* Resolve `join` promise after API channel ready

Previously, the (implicit) promise returned by `join` would resolve
after establishing the transports, but before the API data channel was
attached.  This meant there was a small window of time where the API
data channel was not ready, and trying to use the subscriber API during
that time would fail silently.

This change returns a promise that does not resolve until after the API
channel is established.

* Set Ion connector's `sfu` before join

It's possible for some of the callbacks to fire while awaiting
`sfu.join`, and those callbacks may need to access the connector's `sfu`
property.  So we should set it before we start the join.

Co-authored-by: adwpc <adwpc@hotmail.com>