text: Title: Mongo fail-over during append can leave a Version in an inconsistent state
  Body: We can trigger this assertion:
  
  ```
  ...
    File "/app/AHL/packages/ahl.tickdownsample/1.16.0-py2.7/app/AHL/ahl.tickdownsample/lib/python2.7/site-packages/ahl.mongo-1.297.0-py2.7-linux-x86_64.egg/ahl/mongo/mongoose/store/version_store.py", line 105, in append
      return super(VersionStore, self).append(symbol, data, metadata, prune_previous_version, upsert, **kwargs)
    File "/opt/ahl/app/AHL/packages/ahl.tickdownsample/1.16.0-py2.7/app/AHL/ahl.tickdownsample/lib/python2.7/site-packages/arctic-1.4.0-py2.7-linux-x86_64.egg/arctic/decorators.py", line 50, in f_retry
      return f(*args, **kwargs)
    File "/opt/ahl/app/AHL/packages/ahl.tickdownsample/1.16.0-py2.7/app/AHL/ahl.tickdownsample/lib/python2.7/site-packages/arctic-1.4.0-py2.7-linux-x86_64.egg/arctic/store/version_store.py", line 449, in append
      Append not possible - please call write() to get versions back in sync''')
  ArcticException: version_nums is out of sync with previous version document.
              This probably means that either a version document write has previously failed, or the previous version has been deleted.
              Append not possible - please call write() to get versions back in sync
  ```
  
  in append, as the `version_nums` are updated before the data is actually written.
  