- fix unique constraint parsing for sqlite -- may return '' for name, however
- add test_get_unnamed_unique_constraints to SQLite reflection tests
- add tentative 'changelog' documentation on #3244
- rework sqlite FK and unique constraint system to combine both PRAGMA
and regexp parsing of SQL in order to form a complete picture of
constraints + their names.  fixes #3244 fixes #3261
- factor various PRAGMA work to be centralized into one call