cache refactoring for better plugin friendliness

For legion cache syncing plugin I need API for dynamic cache udp syncing reconfiguration, so I moved few code pieces to dedicated functions and exported few already available functions in uwsgi.h.

But I'm not sure if this is enough to make new udp peers work (list of nodes that will receive updates). Please take a look and if calling ```uwsgi_cache_setup_nodes``` is enough.