Update to bulk order request functionality

Beginning on 20 February at 04:00 UTC, market orders will no longer be permitted in bulk order requests submitted through the API. This includes both orders submitted with ordType=Market and implicit market orders (orders submitted with a side but without a price). After this time, including a market order in your bulk order request will result in the entire request being rejected. This functionality update is part of the ongoing simplification of our API and trading system architecture.