backup nodes in subscriptions

[prymitive]

fixes #179

I'm making pull request since I fear that patch floating somewhere in comments will be lost at some point. This way it's easier to review and comment on changes.
Would be nice if we had such feature at some point, but I don't depend on it.

[prymitive]

would be nice to have in some not very distant future

[unbit]

something to think about for 2.1

[unbit]

i have reviewed your patch and i like the approach. I would like to rewrite the uwsgi_send_subscription function api too to take a struct instead of a potentially growing list of parameters.

[prymitive]

great, do you need me to refactor this patch or you are going to apply some modified version?

[unbit]

i will make some modification but the general logic will remain the same (expecially the 'level' thing)

[unbit]

I am working on it for 2.1. I think algorithms will remain the same. But before merging i will implement the possibility to add more algorithms via plugins. (in this way users could implement specific algos with specific usages of weights and backup_levels)