Simple POW using hashcash algorithm

This is example of using POW (Proof of work) with hashcash algorithm function

For run locally please execute those commands:

    docker build -t pow-server -f server.Dockerfile .
    docker build -t pow-client -f client.Dockerfile .
    docker-compose up 

Why hashcash algorithm was chosen:

It's my first experience in "blockchain and all related to that" so after reading about POW, hashcash and other functions I decided to use hashcash because of:

• It's simplest and most clear for me, I found a lot if docs/guides and even simple lib
• Compare to other functions like Merkle tree or Guided tour puzzle protocol
  • Merkle tree need more calculations on server side, and it's grow with number of leaves and depth
  • For using Guided tour puzzle protocol client should regularly request server about next parts of guide, that complicates logic of contract