provide get_info endpoint

[thepiwo]

• add endpoint to get the clients info, e.g:

{
    "current_block_version" : 1,
    "current_block_height" : 2,
    "current_block_hash" : xyz,
    "genesis_block_hash" : abc,
    "peer_count" : 8,
    "difficulty_target" : 21,
    "public_key": pubkey
}

• replace ping/pong with get_info, only add peer if genesis block has is correct