This patch refactor the ICE transport system used for media streams
by using a dual-buffered like transport:
- one temporary used during the connection establishment
- one used when established and used by other components

When the established is used, the temporary is used to hide all the
connection stuff that can take time to succeed.

This has the (good) side-effect to resolve an issue on holding media
during a call (unhold doesn't work, media using an old transport).

Change-Id: I6b634443e06ece2dec2dd26a4b7ea429319d98d2
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>