Handling of market orders is gross

[SamStudio8]

• Many functions need a reference_price keyword argument which is a bad smell
• The get_book_for_symbol orderbook repo functions have to hack an inf/-inf price into None price orders to allow sorting
• highest_bid and lowest_ask are also a hack
• Market price asset freezes will eventually cause incorrect balances in user accounts (that whole system needs changing anyway)
  • Post-trade update_user uses the orders and not the trade, meaning its harder to update the balances for market orders