position = [
0: currencyName (string)
1: value (Efx::Size)
2: counterpartyId (Efx::ClientId)
3: maxReachablePosition (Efx::Size)
4: minReachablePosition (Efx::Size)
]
// snapshot
['K', feed_id, 'S', [position]]
// new position
['K', feed_id, '+', position]
// update position
['K', feed_id, 'M', position]
// del position
['K', feed_id, '-', position]
// failed to subscribe
['K', feed_id, 'Z', 2]
// unsubscribed
['K', feed_id, 'U', 0]