Fix a typo in the documentation for sqlite3_threadsafe().
Fix a race condition to do with very large index keys in shared-cache mode.
Add the threadtest4.c test program.  Not yet working.
Fix a bug in the threadtest4.c program.  Remove the keyinfo cache as it provides
minimal performance improvements, and then only at SQL preparation time, not
at runtime, and it has problems with data races in shared-cache mode.  We might
later add the keyinfo cache back but only enable it when shared-cache mode
is off.
Merge the cell overflow page number cache thread race fix from trunk.
Make sure the Btree mutex is held when setting the locking mode and the
secure delete flag when attaching a shared-cache database.
Remove the KeyInfo cache (for now - perhaps we will add it back in later - or
maybe not since it provides negligible benefit but adds a lot of complexity
and thread-safety risk).  Add a mutex to ATTACH to deal with a data race.
Add new tests to the threadtest4.c program.  Fix a long-standing data race
in WAL mode for shared-cache.