Require oldestVersion increasing

ConflictSet.h

@@ -42,7 +42,8 @@ struct ConflictSet {
   /// overlapping ranges. Reads intersecting writes where readVersion <
   /// writeVersion will result in `Conflict` (or `TooOld`, eventually)
   void addWrites(const WriteRange *writes, int count);
-  /// Reads where readVersion < oldestVersion will result in `TooOld`
+  /// Reads where readVersion < oldestVersion will result in `TooOld`. Must be
+  /// greater than any previous oldestVersion.
   void setOldestVersion(int64_t oldestVersion);
 
   /// Reads where readVersion < oldestVersion will result in `TooOld`. There are