Recover metadata when nameserver restart in stand alone mode

この issue には説明がありません。

The issue title suggests a problem with metadata recovery when a nameserver restarts in standalone mode. Start by examining the nameserver component and its metadata persistence mechanisms in the OpenMLDB codebase. Look for existing tests related to nameserver restart or metadata recovery to understand the expected behavior. Determine what 'standalone mode' entails and how metadata should be preserved across restarts.

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