mineBlock(difficulty) {
    while (
      this.hash.substring(0, difficulty) !== Array(difficulty + 1).join('0')
    ) {
      this.nonce++;
      this.hash = this.calculateHash();
    }

    debug(`Block mined: ${this.hash}`);
  }
Starts the mining process on the block. It changes the 'nonce' until the hash
of the block starts with enough zeros (= difficulty)

@param {number} difficulty