order - extra info

[ghost]

we need to add the following to the order:

• timestamp
• priority_id (uint64_t) - sometimes it's = timestamp, some venues assign it differently. for ITCH it's a copy of timestamp
• flags - for example if we know that it's ours order we can set FLAG_OWN or FLAG_IMPLIED
• filled_quantity - for partially filled orders it may be relevant to know
• user_data - opaque pointer