simplify bidder assembly process

It looks like coding a new bidder not based on Geo model we provided in examples requires quite few lines of code. the pattern we observed is the following - output from one matcher becomes input to the next in the chain.

Following needs to be implemented to simplify bidder assembly process:

Create chained_slector to delegate output from previous matcher to the next in the chain
Create generic version of bidder cache loader "GenericBidderCacheLoader" capable of loading all matchers data

Same GenericBidderCacheLoader should be utilized to retrieve matched data from caches

template<typename Entity, typename ...Entities>
struct GenericBidderCacheLoader<Entity, Entities ...> : GenericBidderCacheLoader<Entities ...>

Simplify the use of bidder as following

.auction_async([&](const BidRequest &request) {
        thread_local vanilla::Bidder<DSLT, Selector> bidder(std::move(Selector()));
        return bidder.bid(request,
                          std::make_tuple(
                              request.site.get().ref,
                              retrieve_referer_f,
                              retrieve_ico_campaign_f,
                              retrieve_campaign_ads_f
                          )
        );
    });

If following implemented then this code

std::make_tuple(
      request.site.get().ref,
      retrieve_referer_f,
      retrieve_ico_campaign_f,
      retrieve_campaign_ads_f
)

Can be easily generated based on data model provided by DSP.

venediktov / vanilla-rtb

simplify bidder assembly process #74