Update a test case in wal2.test that explicitly corrupts a checksum in the wal file to account for the fact that the first byte of said checksum may initially be 0xFF.
Add #ifdefs for test coverage.  Add a testcase().
Fix an obscure problem to do with temp register allocation that could occur if more than one simple SELECT within a compound SELECT uses a partial sort.
Add a comment explaining why WhereLoop cost adjustments are omitted for
skip-scan loops.