readVarInt

from:
https://github.com/conformal/btcwire/blob/master/common.go

// readVarInt reads a variable length integer from r and returns it as a uint64.
func readVarInt(r io.Reader, pver uint32) (uint64, error) {
	b := make([]byte, 8)
	_, err := io.ReadFull(r, b[0:1])
	if err != nil {
		return 0, err
	}

	var rv uint64
	discriminant := uint8(b[0])
	switch discriminant {
	case 0xff:
		_, err := io.ReadFull(r, b)
		if err != nil {
			return 0, err
		}
		rv = binary.LittleEndian.Uint64(b)

	case 0xfe:
		_, err := io.ReadFull(r, b[0:4])
		if err != nil {
			return 0, err
		}
		rv = uint64(binary.LittleEndian.Uint32(b))

	case 0xfd:
		_, err := io.ReadFull(r, b[0:2])
		if err != nil {
			return 0, err
		}
		rv = uint64(binary.LittleEndian.Uint16(b))

	default:
		rv = uint64(discriminant)
	}

	return rv, nil
}