Implement collector() methods for flow()

[tshemsedinov]

Make flow() compatible with collector(), implement methods:

• [x] collect(key, error, value)
• [ ] pick(key, value)
• [ ] fail(key, error)
• [ ] take(key, fn, ...args)
• [ ] timeout(msec)
• [x] done(callback)
• [x] cancel(err)
• [x] then(fulfilled, rejected)