Save known.met as soon as a file is hashed, and not just when you shut down the app.
Two bugs with this: one is when you crash you have to rehash everything, but a worse bug is: I clicked reload several times, and the hashing thread hashed the same files over and over for each click of reload.
If each file is saved as soon as it's hashed, if a reload is started again it will skip it since it's already hashed, and will also preserve the hashes in case of a crash.
