Issue 43 Caching Market API calls - AdSlot & AdUnits

[elpiel]

Resolves #43

CHANGELOG:

• MarketApi is now wrapped in an Arc for cheap cloning

• Generic Cache implementation for caching records for /units-for-slot route

  • Implementations for the generic Cache (mod supermarket::cache::market), caching the following calls:

    • GET AdSlot from Market - mod supermarket::cache::market::ad_slot

      • type AdSlotCache = Cache
      • impl ClientLike for AdSlotClient with calls via the MarketApi

    • GET AdUnit from Market (AdSlot's fallback unit) - mod supermarket::cache::market::ad_unit

      • type AdUnitsCache = Cache
      • impl ClientLike for AdUnitsClient with calls via the MarketApi

    • GET AdUnits (fetched by Ad Type from the Market) - mod supermarket::cache::market::ad_unit type AdTypeCache

      • type AdTypeCache = Cache
      • impl ClientLike for AdTypeClient with calls via the MarketApi

• mod supermarket::cache moved to supermarket::cache::campaign - the Cache implementation for Campaigns is now moved to separate, own module

• struct Caches (mod supermarket::cache) implementation that contains all the cache implementations in 1 struct

TODOs:

• Way to clean all Caches from struct Caches
• Mocked clients for AdSlotCache, AdUnitsCache, AdTypeCache
• Simplify tests by creating Mocked clients for AdSlotClient, AdUnitsClient & AdTypeClient instead of mocking the API calls in /units-for-slot tests (units_for_slot_tests.rs)

[elpiel]

I just couldn't figure out a way for K to be able to support owned and reference values... I've tried mimicking DashMap and HashMap, but I got beaten by async and the async_trait crate