Speed up field queries

On my laptop, this simple change speeds up
FieldValues.get_search_values by hundreds of times, finishing in, say,
89 msec instead of 46 seconds.  It may still be too slow for really
high-cardinality fields (3.5 sec for 263K values - borderline).  The
get_random_values method is enhanced in the same way.

Signed-off-by: Kevin Murphy <murphyke@email.chop.edu>
Merge pull request #181 from murphyke/issue-180

Speed up field queries