[PATCH] WCNG: Enforce that a pristine checksum always references an entry in the pristine store.

I'm working towards cleaning up the pristine store automatically.

Although not strictly necessary, I think it will be helpful if a
pristine text checksum in the DB shall be guaranteed always to reference
a pristine text that is currently in the store. In support of this
goal, the current patch enables SQLite checking for that constraint (no
effect in sqlite versions < 3.6.19) and adjusts the 1.6-to-1.7 upgrade
code to follow that rule.

I think I only made minor edits since it last passed all tests.

Any feedback before I commit it? Philip, you probably know the upgrade
code best. How does this look?

- Julian