Support bootstrapping for nodes when starting

[mhchia]

What was wrong?

https://github.com/ethresearch/sharding-p2p-poc/issues/31

How was it fixed?

• Add the flag -bootstrap to specify if the node is going to start the dht.Bootstrap process, which keeps trying to ask random peers from its peers.
• Add the flag -bootnodes to specify the nodes to connect and bootstrap with. The

Example: specify that we want the node to bootstrap, and bootnodes are /ip4/127.0.0.1/tcp/10000/ipfs/QmS5QmciTXXnCUCyxud5eWFenUMAmvAWSDa1c7dvdXRM and /ip4/127.0.0.1/tcp/10001/ipfs/QmexAnfpHrhMmAC5UNQVS8iBuUUgDrMbMY17Cck2gKrqeX. The format of an address should be Human-readable multiaddr, so basically in our case is /ip4/{ip}/tcp/{port}/ipfs/{peerID}

./sharding-p2p-poc -bootstrap=true -bootnodes=/ip4/127.0.0.1/tcp/10000/ipfs/QmS5QmciTXXnCUCyxud5eWFenUMAmvAWSDa1c7dvdXRMZ7,/ip4/127.0.0.1/tcp/10001/ipfs/QmexAnfpHrhMmAC5UNQVS8iBuUUgDrMbMY17Cck2gKrqeX

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[mhchia]

cc @ChihChengLiang