* fix(api): make stick sessions actually work and make them non-racy
We apparently have a way to specify that all "related" requests should
go to the same node. However:
1. It didn't work at all. All future requests would go to the first successful
node from the first request. Because that's how stack variables work.
2. It was racy if the context was re-used concurrently. But only the
first time, see point 1.
* test(api): test the API merge proxy
1. Test whether or not it works.
2. Test stickiness.
* fix(api): update OnSingleNode documentation
