Merge or update the fork with upstream 0.15.x line

[prlanzarin]

Don't know if a merge is warranted or even possible. It might be easier to just switch to 0.15.x cold turkey, port over the RegisterContext changes and try to make it work over WS in Node.js.
Things to watch for:

• Make sure we can do multiple alias REGISTER requests within a single UA.
• Make sure there are no regressions with the INFO nicst transactions state acceptance.

If we make it work with 0.15.x over WS, I suggest dropping the UDP and TCP transports and rely on Kamailio to do the transport protocol conversion. We might keep our 0.7.x fork as a dev dependency to use the UDP/TCP transports to ease development, but that won't be recommended in order to maintain behavioural parity between dev and prod environments.