[Feature] persist DHT of bootnode

[LeoHChen]

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Problem/limitation At Hand

The current bootnode doesn't persist DHT. So, if we restart the bootnode, the DHT will be lost. Since we are using DHT of libp2p, we need to find a way to save the DHT to disk.

Proposed Solution

Bootnode should save DHT automatically, and reload it the DHT if it exists. If you are familiar with libp2p, this may be an easy task. Some investigation is needed if not familiar with libp2p.

Alternatives Considered

A clear and concise description of any alternative solutions or features you've considered.

Additional Context

Add any other context or screenshots about the feature request here.

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Issue Status: 1. Open 2. Started 3. Submitted 4. Done

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[touhonoob]

depends on libp2p/go-libp2p-kad-dht#315

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Issue Status: 1. Open 2. Started 3. Submitted 4. Done

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