MB-49207: Set cas on temp items

So that we can use the cas as a "token" to determine if the item
in the HashTable at the point of the completion of the BgFetch is
the same as the item that originally drove the BgFetch.

Change-Id: Iff7b9118f6839b7d7590e189c46c6650385c3dff
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/165105
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>

engines/ep/src/vbucket.cc

@@ -3875,6 +3875,12 @@ VBucket::AddTempSVResult VBucket::addTempStoredValue(
 
     updateRevSeqNoOfNewStoredValue(*v);
 
+    // MB-49207: We set the cas of temp items so that when we complete a BGFetch
+    // we can check that we are replacing the correct temp item with the one
+    // that triggered the BGFetch. If we didn't then we'd run the risk of
+    // fetching older revisions of documents back into memory.
+    v->setCas(nextHLCCas());
+
     return {TempAddStatus::BgFetch, v};
 }