Take into account peer state for standard API

[michaelsproul]

Lighthouse nodes currently have their peer count overstated on eth2stats.io because eth2stats-client ignores the peer states when reading /eth/v1/node/peers. I think it should use the state field to work out which peers are connected.

E.g. eth2-stats reports this number:

$ curl -s "http://localhost:5052/eth/v1/node/peers" | jq '.data | length'
244

when it should report:

$ curl -s "http://localhost:5052/eth/v1/node/peers" | jq '.data | map(select(.state == "connected")) | length'
50

[michaelsproul]

Related ethereum/beacon-APIs#101 (adds peer filtering by state to the API)

[protolambda]

implementing this now