This fixes the following scenario:
1. the master sends invalidations to clients,
   and unlocks to storages  (oid1, tid1)
2. the storage receives/processes the unlock
3. the client asks data (oid1, tid0)
4. the storage returns tid1 as next tid, whereas it's still None in the cache
   (before, it caused an assertion failure)
6. the client processes invalidations