Thread pool implementation

Link to the task

Rust book: https://doc.rust-lang.org/beta/book/ch20-02-multithreaded.html

Idea

Firstly implemented for using in web-server, that can respond to multiple requests at once. Limited workers, so that it safe against DoS attacks. Implemented graceful shutdown so that if ThreadPool goes out of scope, it still finishes last jobs - but stops receiving new ones. Also implemented error handling and own errors. If any error occurs within dependant modules (std::mpsc, std::thread) error will be converted / mapped to ThreadPoolError type. This type implemented Display trait, so it can be printed out

Dependencies

• thread - standart library implementation of threads
• Arc and Mutex - to enable locking mechanism between threads, one sender and multiple receivers can work parallel