Find less memory intensive way to indicate LUI open but not configured.

[finson]

The LogicalUnitInfo constant object NULL_LUI may be wasting memory. Verify and replace if appropriate.

[finson]

In each LogicalUnitInfo object there is information about two available timers. For each timer, there are two 32-byte Firmata message buffers and four unsigned long (4-byte) integer time values, plus a few miscellaneous values. 2*(232+44) = 160 bytes per object.