Support for AZ awareness

[orgalaf]

How about adding a feature whereby components will be able to prefer counterparts who are closer to them. For instance a requester will prefer responders in the same AZ over responders in other AZ's.

[dashersw]

This would be a great feature. In fact, cote already has an undocumented feature called subsets. If you look at requester.js code, you'll see that messages can be routed to a subset of all available responders, configured via a __subset property in the message. Likewise, responders can decide which subsets they are in via a subset property in their advertisement message. So one can set these configurations via environment variables, where an environment variable SUBSET would point to an AZ, therefore requesters would prefer this subset. The current implementation lacks one feature though — fallback. If you don't have any items in the subset, the request enqueues instead of being routed to the other responders.