Interface change! Return TooOld after 2e9 versions

Event if setOldestVersion wasn't called

include/ConflictSet.h

@@ -54,6 +54,8 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
     /** `end` having length 0 denotes that this range is the single key {begin}.
      * Otherwise this denotes the range [begin, end) */
     Key end;
+    /** `readVersion` older than the the oldestVersion or the version of the
+     * latest call to `addWrites` minus two billion will result in `TooOld` */
     int64_t readVersion;
   };
 
@@ -141,6 +143,8 @@ typedef struct {
   /** `end` having length 0 denotes that this range is the single key {begin}.
    * Otherwise this denotes the range [begin, end) */
   ConflictSet_Key end;
+  /** `readVersion` older than the the oldestVersion or the version of the
+   * latest call to `addWrites` minus two billion will result in `TooOld` */
   int64_t readVersion;
 } ConflictSet_ReadRange;