Prebid Server returns exp rather than ttl (prebid#5715)

Updating how pbsBidAdapter sets the "TTL" for bids. 

TTL in PBJS terms is how long the bid can stay in cache. However, the OpenRTB standard location for this value is `exp`.

Author: bretg

modules/prebidServerBidAdapter/index.js

@@ -825,9 +825,9 @@ const OPEN_RTB_PROTOCOL = {
           bidObject.meta = bidObject.meta || {};
           if (bid.adomain) { bidObject.meta.advertiserDomains = bid.adomain; }
 
-          // TODO: Remove when prebid-server returns ttl and netRevenue
           const configTtl = _s2sConfig.defaultTtl || DEFAULT_S2S_TTL;
-          bidObject.ttl = (bid.ttl) ? bid.ttl : configTtl;
+          // the OpenRTB location for "TTL" as understood by Prebid.js is "exp" (expiration).
+          bidObject.ttl = (bid.exp) ? bid.exp : configTtl;
           bidObject.netRevenue = (bid.netRevenue) ? bid.netRevenue : DEFAULT_S2S_NETREVENUE;
 
           bids.push({ adUnit: bid.impid, bid: bidObject });