datachecksums: handle invalid and dropped databases during enable

Hello

While testing the online enabling of datachecksums, I discovered some
corner cases that can cause the background process to fail:

* Invalid databases were only discovered when we tried to create
checksums for them, when we could instead discover them before
starting and fail early
* Databases dropped while the checksum process was running also
resulted in errors when it reached them and tried to build checksums
for them. In this case we could continue safely: the database was
dropped, there's no need to create checksums for it.

I attached a patch that adds testcases for these and fixes the issues.