Simplication and optimization of error message handling.
Avoid unnecessary calls to sqlite3_free() from within sqlite3VXPrintf().
Enhance the pcache1PinPage() routine so that it called much less often and
runs much faster in the cases when it is actually called.
Create and use a new pager interface sqlite3PagerUnrefNotNull() that works
just like sqlite3PagerUnref() but guarantees that its argument is not a
NULL pointer.
Reduce the number of calls to the subjRequiresPage() routine inside of pager.
Performance optimizations in the pager_write() routine of pager.c.