Skip local sync of files in progress

[nlmarco]

If the sync is interrupted (network error, power failure, user pressing Ctrl+C, other reason), a (maybe very big) file might still be inProgress.

When we resume the sync, we currently generate hashes for all files, including those being inProgress. This is unnecessary (they are not replicated to any other repo, anyway) and consumes a lot of time (if the file is big).

We should thus skip files that are inProgress in the local sync.

[nlmarco]

We should then - of course - update the persistent _FileChunk_s already directly when a chunk is written. Otherwise the next sync would not skip the stuff that is already done and instead start from scratch again.