Experiments in picking better query plans, especially when the usage of one
index is a subset of another.
Make sure that an index that covers a proper superset of the WHERE clause
terms of some other index has a lower cost than the other index.
Also make sure an index that is a proper subset of some other index has a
higher cost than that other index.  Add test cases.
Remove an unnecessary conditional.
Ensure the "PRAGMA journal_mode=WAL" works coming from any other journal_mode
with ATTACH-ed databases.
Merge changes to the query planner that strive to ensure that any index 
usage that is a proper subset of some other index usage always has a slightly
higher cost.
Merge the latest trunk changes into the threads branch.
Fix typo in a Windows threading support routine.
Fix harmless compiler warnings.