Add serial-types 8 and 9 for integer constants of 0 and 1 that use zero

Add serial-types 8 and 9 for integer constants of 0 and 1 that use zero
bytes of storage.  Make the default file format 4.  Add the
SQLITE_DEFAULT_FILE_FORMAT compile-time option to lower the default
file format number so that newly created databases can be read and
written by older versions of SQLite. (CVS 2845)

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