Omit the uneven=N argument in sqlite_stat1 and replace it with var=N1,N2,...

Omit the uneven=N argument in sqlite_stat1 and replace it with var=N1,N2,...
so that the variation in each column is reported separately.  Omit the "slow"
argument from sqlite_stat1, computing that flag internally.  PRAGMA stats is
enhanced with a new column "est" that shows the aiRowLogEst array for each
index after it has been modified by "var=..." and also the "slow" flag.

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