In sqlite_stat1, revert the number of rows to the average, not adjusted for

In sqlite_stat1, revert the number of rows to the average, not adjusted for
the maximum.  Instead add "uneven=NNN" where NNN is a multiplier to apply to
the average to get the maximum.  "uneven" is only reported if NNN is 10 or more.
Also add the "slow" argument is the maximum number of rows reported by a
full equality match is so large that a table scan seems likely to be faster.

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