provide debug info when collection corrupt
Merge branch 'master' of github.com:dae/anki
new media syncing protocol

- media syncing no longer locks the account, so it can be done
  in the background in the future, and multiple clients can safely
  sync media at the same time
- all operations are now idempotent, so they can be repeatedly safely
  in the event of a connection error
- whether it's a normal incremental sync, an initial sync,
  or the media database has been deleted, no files will be uploaded
  or downloaded if they already exist on the other side
- file removals are now chunked like additions & updates, preventing
  timeouts due to large requests
- if the server can't process a chunk in time, it will return a count
  of what it did process, so the client can retry the rest

Notes for AnkiDroid:

- when porting this, recommend you pick a different name for the
.media.db2 file, so users don't accidentally copy the AD version to
the desktop or vice versa
- please make sure filenames are added to the zip in NFC form