Performance chunkchecksum in batches

Fix for: TODO [high] Performance: This queries the database for every chunk, SLOWWW!

Anecdotal running of testManySyncUpsAndOtherClientSyncDown suggests that this solution indeed does result in better performance.

In addition, this pull request contains some alterations to the UploadInterrupted test. This still was failing weirdly, partly dependent on whether the checksum of a multichunk contained "db" or not. I took the liberty of adding a TODO there to write tests which actually test something specific, instead of testing so broadly and arbitrarily.