Add PeerInfo() to get a list of connected spv peers

[beansgum]

PeerInfo() returns information about currently connected spv peers in json.

[
    {
        "id": 1,
        "addr": "127.0.0.1:19108",
        "addr_local": "127.0.0.1:53402",
        "services": "00000005",
        "version": 8,
        "sub_ver": "/dcrwire:0.4.0/dcrd:1.7.0(pre)/",
        "starting_height": 559536,
        "ban_score": 0
    }
]