IO events were handled by a unique thread owned by IceTransportFactory.
This patch changes that by creating a thread per IceTransport instance.

This improves a bit (on multiple-threaded architectures at least)
events handling as not all events handler run at the same speed
(SIP events are slow, Media IOs need low overhead).

Issue: #79692
Change-Id: Ie742373176f9447a14286910e6af562427cecdbd