Implement peer identity

[matjazv]

Description

Each peer needs to have a unique and fixed peer ID. On boot if a peer doesn't have a peer ID, generate and save it for a later use.

Consider a possibility to import private key via config file. Than something like this can be used: privKey, err := crypto.UnmarshalPrivateKey([]byte)

Acceptance Criteria

• Peer has a fixed peer ID and is not changed during a system reset.
• All new code is unit tested.

Additional Information

• seed should be given from config
• crypto.GetKeys can be used for generating key pairs
• Keypair can be generated like here: https://github.com/libp2p/go-libp2p/blob/master/examples/libp2p-host/host.go#L43-L46
• Option for a peer while creating it: https://github.com/libp2p/go-libp2p/blob/master/examples/libp2p-host/host.go#L63