Deploy TPL Dataflow/Actor

[johanneszab]

In the AbstractDownloader.cs and *Crawler classes, use something like TPL Dataflow/Actor model instead of creating a huge list of Tasks for tracking and semaphores for pressure management. Next to being less error prone (improper exception handling in the downloader/crawler might lead to a stall) it should be more convenient to use.

Initially, TumblThree was one of my toy projects to "learn a new language", a try to build a nice open source application without spaghetti code (i.e. usage DI, MVVM, "clean code", etc) and once in my life setup a new project from scratch. As such, I wanted to use as few third party libraries as possible with the additional goal of implement as much as possible by myself using the raw .NET framework.