Avoid temporary by-value copies of std::shared_ptr<BidRequest> during Au...

...ction injection.  Auction correctly stores a by-value copy of std::shared_ptr<BidRequest>.

I believe this should be a safe optimization.  Please review js/rtb_router_js.cc@314: 'return args.This();'

I'm not familiar with this idiom, if @270: 'std::shared_ptr<BidRequest> request;' is guaranteed to outlive the call to router.injectAuction(...) then this should be a safe commit, otherwise, it should be rejected.