Improve lookup for structure market orders with future ESI endpoints

[TheJokr]

ESI allows authenticated searches to find structures (much like the in-game search function) and also gives access to structure market orders. This can be used to add market orders of accessible structures to get_market_orders.
Citadel spam is a problem with that approach though, since market data for every citadel needs to be requested individually. We will need to make those requests in parallel (see futures and their backport).

[TheJokr]

I'll wait for CCP to add the X-Pages header to the structure market orders route before continuing with this.

Edit: This is live now (12bc8f0), but there's no way to check if a structure has a market module installed without potentially producing a 403.
Edit 2: An endpoint to query structure service data is supposed to be added sometime after parity is reached.

[TheJokr]

It seems like CCP stopped most development work on ESI, so that endpoint will probably never become reality.
However, I had added a bespoke cache for market structures in c808458 already. The cache remembers a structure's location, type ID (i.e., can it fit a market module?), and whether the previous market lookup was successful. The usual structure spam can't fit market modules, so we immediately save ourselves a lot of requests.

Only new and expired entries can now produce 403s, which reduces their number to a level that (usually) doesn't trigger error rate limiting. I'll keep this issue open in case CCP ever adds a way to query the required information directly.