Sanity check on server

To get a more bulletproofed upload process the server should do some checks to ensure that we always have a valid and consistent folder with the correct metadata and encrypted files:

• file size
  • on lock the clients tell server the file size
  • an unlock the server checks if the correct file size was transferred, otherwise revert and send error to client
• file check
  • although the server cannot read the entire metadata file, it can check & ensure that the count of encrypted files matches the stored files in the folder
  • on unlock: check if correct file count and file names are stored both in metadata and in local folder

While this should always be correct (as the clients should handle it correctly), with these enhancements we can make (more) sure that every upload is consistent.
The only potential problem (I can imagine) then is that the clients uploads a "wrong" playload, e.g. you cannot decrypt it anymore.

--> with this we check for

• file size
• file count
• encrypted file name
• #20: allows to revert a single upload to a consistent state

@MorrisJobke (as we discussed it)

The payload does not name files or tests. Start by locating the server's upload lock/unlock handlers and the metadata and folder validation path; compare the requested size, file-count, and encrypted-name checks with issue #20's revert behavior. Done means inconsistent uploads are detected, reverted to a consistent state, and reported to the client.