thread safety: KV::Store::clear() requires version spinlock

[douglasism]

Description

kv.h::Store::clear needs a version spinlock

      std::lock_guard<SpinLock> vguard(version_lock);

    void clear()
    {
      std::lock_guard<SpinLock> mguard(maps_lock);

      // This deletes the entire content of all maps in the store.
      for (auto& map : maps)
        map.second->lock();

      for (auto& map : maps)
        map.second->clear();

      for (auto& map : maps)
        map.second->unlock();

<<<<<<<<<<<<<<<<< missing version_lock >>>>>>>>>>>>>>
      version = 0;
      compacted = 0;
      last_replicated = 0;
      last_committable = 0;
      rollback_count = 0;
      pending_txs.clear();
    }

[achamayou]

@douglasism this call is only used in tests, but you're absolutely right!