* ref(JingleSessionPC): add local tracks with offer/answer

Refactor the current code to add local tracks together with initial
offer/answer to make this atomic/single operation on
the modificationQueue.

This is required to be able to get rid of 'delayedIceCandidates' list
and to execute 'addIceCandidate' task on the queue. If local track
addition is not bundled with initial offer it will often happen that ICE
candidates are queued, before the offer/answer which is queued only once
the local tracks task is done.

* ref(JSPC): use queue for ICE candidates

Refactors the code to get rid of 'delayedIceCandidates' buffer and
use the modification queue to synchronise with the initial offer/answer.

* doc(JingleSessionPC): improve docs