Make the maxCount calculation lazy for faster startup

[tokee]

In order to minimize memory usage for facet counters, a full facet count is performed upon index change. For large indexes this introduces a noticeable delay. The maxCount is only needed when using packed counters, so lazy evaluation would ensure the penalty is only paid when necessary.

[tokee]

In order to properly guess if the result will be sparse, a full run through of the ordinal counts is necessary to calculate the number of references. maxCount is a nearly free add-on to that analysis.