Use identity instead of sid where possible

[keepingitneil]

Hrmm ok, @theomonnom and I spoke offline about keying all local participants off identity and removing the participants_by_identity mapping on the python side which would require using identity for the ffi.

Are you saying that there is logic that requires participant_sid (due to out of order events maybe?) in cases where the identity is the same?

[theomonnom]

is the livekit crate using sid?