RxJS: important link for rxjs.

1. https://blog.kwintenp.com/how-share()-can-reduce-network-requests/
2. https://coryrylan.com/blog/angular-multiple-http-requests-with-rxjs
3. https://www.learnrxjs.io/operators/multicasting/share.html
4. https://coryrylan.com/blog/angular-multiple-http-requests-with-rxjs
5. http://plnkr.co/edit/L2eHKy2mZg4oTwxL6PMy?p=preview (important)
6. https://www.learnrxjs.io/operators/combination/combineall.html (tutorial)
7.