Slight refactor of auto-relay interfaces

[ghost]

Following some synchronous discussion in the libp2p bi-weekly meeting today, resolved:

• autorelay #454 is good to merge 🍾
• Ideally, interfaces that may change should be marked as unstable in that PR
• @vyzo, @Stebalien and @bigs should immediately figure out how to refactor interfaces before users start relying on them. Two main areas:
  • push identify method
  • revise the identify service to push new addresses

I'll let you all discuss these matters here. Marking as high priority so that it does not get forgotten. 😀

[raulk]

Closing as this is a duplicate of #472 – the latter of which is tagged and all.