Adjust the output row estimating logic when using STAT2.
Allow the sqlite_stat2 table to contain a number of samples other than
the default of 10.
Add a column to the sqlite_stat2 table that contains the number of entries
with exactly the same key as the sample.  We do not yet do anything with this
extra value.  Some tests in analyze2.test are failing.
Bug fixes to the sample-count logic for STAT2.  A few test cases added.
Fix a compiler warning when STAT2 is off.  More test cases.  Fix legacy tests
to deal with the new STAT2 logic.
Merge together the fork in the query-planner-tweaks branch.
The sqlite_stat2.cnt field is parsed if it is present.  But it is not yet
used.  A large comment added to analyze.c to explain the format of the
ANALYZE system tables.
The ANALYZE command adds the sqlite_stat2.cnt column if it does not already
exist.
Add code to actually use the sqlite_stat2.cnt field in the query planner.
This changes some plans resulting in a few failures in analyze5.test.