Begin a series of changes designed to reduce the scope and frequency of

Begin a series of changes designed to reduce the scope and frequency of
invalidating schemas.  Design goals are that the internal schema should
never be deleted out from under a prepared statement that is running and
that all prepared statements should be expired if the schema is invalidated.
At the same time, minimize the number of schema invalidations.  This change
merely revises the sqlite3ResetInternalSchema() function to use -1 as the
wildcard for "all" rather than 0, so that we can reset the main schema
independently of all the others.

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