Writing tmp files

[m-gl]

DAQSnapshotService is accessing the file that DAQAggregator has not yet closed (DAQAgg is still writing to it). This results in file being not de-serialized (and stacktrace in logs) in that iteration of SnapshotService (unexpected end o file). However it's discovered again and successfully de-serialized a moment later when the file is ready. This effect is not visible in DAQView as all snapshots are delivered sooner or later. It could be easily fixed by writing to tmp file that DAQSnapshot is filtering and then renaming it. For example:

1. writing to snapshot.json.gzip~ (with suffix)
2. renaming to snapshot.json.gzip
3. reading only files without suffix '~'