Remove direct usage of unexported fields in ICETransport

[masterada]

There are some direct usages of unexported fields of ICETransport
from non ice-related methods. This would be problematic when ice
once ice related code is moved to a separate packet. Added proxy
methods to ICETransport to avoid this.

Reference issue

Rel #646

[Sean-Der]

Hey @masterada

Want to tag+bump pion/ice and rebase off of that? This has that annoying PreICE fail :/

[masterada]

you got to be kidding me :D

[Sean-Der]

@masterada correct me if I am wrong, but this feels a bit tech debty to me. I think this would possibly be the right solution (and I can help!)

• Move ICETransportState to a atomic. Then we don't have to worry about mutex juggling
• Fix pion/ice to properly emit state

[masterada]

atomic - good idea
fix pion/ice - already created an issue for that and assigned myself, although im not sure when i will have time for it, not sure how hard it will be to write tests for it

there is still a question: do we need to make sure that once Close() call returns, State() will return closed (because if we wait for the agent's event to set the closed state for the transport, this will not be the case)

[Sean-Der]

One minor change! Otherwise LGTM!