copied from https://fedorahosted.org/389/ticket/48114

Allow setting which event backend to use in ns_thrpool_new e.g.
{{{
struct ns_thrpool_config ns_config;
ns_thrpool_config_init(&ns_config);
ns_config.event_backend = "tevent";
tp = ns_thrpool_new(&ns_config);
}}}
to tell nunc stans to use tevent as the event backend.

This is similar to the way the event frameworks allow you to specify to use epoll, poll, select, etc. as the polling mechanism.