Feature/runset

[db7]

Introduce runsets to control the lifecycle of views and processors (runnable interface).

Here is an example of how to use runsets:

// create processors and views (they are runnables because have Run(ctx) method)
p, _ := NewProcessor(brokers, DefineGroup(group, Input(topic, codec, cb)))
v, _ := NewView(brokers, topic2, codec)

// start runnables creating a Runset.
rs := Start(p,v)

// wait for bad things to happen
wait := make(chan os.Signal, 1)
signal.Notify(wait, syscall.SIGHUP, syscall.SIGINT, syscall.SIGTERM)
select {
case <-rs.Done(): // wait for one of the runnables to return
case <-wait:      // wait for SIGINT/SIGTERM
	rs.Stop() // gracefully stop runnables
}

// wait for all runnables to return and collect error messages
if err := rs.Wait(); err != nil {
	log.Fatalln(err)
}