• Fix the datamodel: it is not possible to store data in intermediate nodes
  anymore (#209)
• Fix serialization of slices (#204)
• Do not fail silently when the synchronisation fails (#202)
• Fix a race in the HTTP backend between adding a watch and updating the store.
  In some cases, the watch callback wasn't able to see the first few updates
  (#198)
• Fix a race for all the on-disk backends between adding a watch and updating
  the store. This is fixed by making Irmin.Private.Watch.listen_dir and
  Irmin.Private.Watch.set_listen_dir_hook synchronous.
• Update the tests to use alcotest >= 0.4. This removes the dependency towards
  OUnit and nocrypto for the tests.
• Make the file-locking code a bit more robust