New concurrency model for core

New concurrency model for Crawljax-core.

- Less code using best practices and correct frameworks
- Tested on 80 sites so far resulted in a **460% higher coverage on average**
- On average, it is **3 seconds per state faster**.
- Crawl overview now also reports exit status (exhausted, max states/time, errored).
- Crawl overview is now serializable and de-serializable for better analysis.
- Fixed the plugin executions

Fixes #102, #237, #238, #159