Extract and deprecate polyamorous mode

[gdamore]

Pair v1 Polyamorous mode was a design mistake. It adds complexity to the pair protocol, while bringing very little value. It can't operate over proxies, and it hurts performance.

What is really needed is a mesh protocol which should support broadcast, unicast, and route discovery for directed sends.

It is possible to mostly replace designs using pair v1 with surveyor pattern, where the monogamous peers send extremely long (infinite?) surveys, and the polyamorous peers reply as respondents. It might require a little more application complexity to do so, and probably raw mode would help. The advantage of such approaches are that they can interoperate with all nanomsg implementations, and support operation over proxies.

In the meantime we should move deprecate the mode, and move it to its own implementation so that pair1 optimizations can proceed unimpeded.

The option should be deprecated, and a new nng_open_pair1_poly() function provided for now.

Once we have a better mesh replacement, the pair1 mode can be removed.