Features/163 buy now

[alexey-frolov]

• Owner defines "Buy Now" price when puts Corgi to the Marketplace
• When anyone bids at least "Buy Now" price auction ends immediately and clearance happens.
• Add 6 test cases:
  1. add_item_for_sale_with_buy_now_price
  2. buy_now_bid_trigger_clearance
  3. buy_now_bid_into_two_steps_trigger_clearance
  4. buy_now_not_reach_then_auction_continue
  5. auto_clearance_return_bids_to_participants
  6. bid_after_buy_now_should_panic