cql: introduce update_parameters::make_tombstone_just_before()

Addresses a quirk in how collection tombstones are compared vs. cells:
a timestamp tie goes to the tombstone.  Matches origin.

cql3/update_parameters.hh

@@ -94,6 +94,10 @@ public:
         return {_timestamp, _local_deletion_time};
     }
 
+    tombstone make_tombstone_just_before() const {
+        return {_timestamp, _local_deletion_time - gc_clock::duration(1) };
+    }
+
 #if 0
     public RangeTombstone makeRangeTombstone(ColumnSlice slice) throws InvalidRequestException
     {