attempt to fix "first snapshot is not read-only" issue

[kostgr]

It did a work for me, but the first snapshots, which was synchronized was not the correct one. Some snapshots were skipped. I've modified each()-Method of Snapshot class. It would be better to skip a writable snapshot and take the next one at the time of synchronization. But I couldn't find a correct place for the check quick enough.

[doudou]

Hi. Thanks a lot for the report & fix !

I don't completely understand the issue there still have with your solution.

The synchronization lists all snapshots, sorts them and then decides what to synchronize (in LocalSync#sync). Filtering them in each_snapshot_raw or later during the sync should make not difference. Moreover, I really don't understand what would be the rationale for a non-readonly snapshot (a.k.a. that snapshots are readonly would definitely be an assumption in the snapsync codebase), so fitlering during the enumeration sounds OK to me.

There are some style fixes needed in your code, but I can do them.