Move logic to request ice servers into factory class

[noeplease]

We would like to remove the functionality use retrieve a local list of ice servers and replace the logic with factory class to allow one to retrieve a list of ice servers from any service.

[noeplease]

I would like to mentioned the reason for the pull request is to seamlessly apply/incorporate Xirsys as a source for turn and stun servers.
Here's an example of what a third party factory would resemble:
https://github.com/xirdev/signalmaster/blob/xirsysIceFactory/xirsysIceServers.js