Avoid copying of versioned files

[varmar05]

In case of geodiff supported files we should completely avoid copying files since they can be open with some clients in WAL mode and thus changes would be lost.

We need either to flush changes right before copy or not to copy at all (might be impossible in some cases). Also we need to investigate how robust is geodiff sync against data loss when some file is still open by 3rd client (e.g. modified between pull and push)

[varmar05]

We currently trigger sqlite checkpoint manually before push. Also we download geodiff changeset files whenever possible.
However, there are still some use cases with potential issues, e.g. changing table schema which geodiff can not handle, therefore we need more actions, e.g.:

• close qgis layers before sync (short-term)
• check existence of WAL files before pull and raise exception
• create sync transactional so we can abort whole pull process if needed or retry individual failures (Retry for failed pull/push requests  #6)

[wonder-sk]

There is SQLite API for copying sqlite databases which should be hopefully safer than verbatim copies and dealing with checkpoints/vacuum: https://sqlite.org/backup.html

[wonder-sk]

The new geodiff 1.0 will have GEODIFF_makeCopySqlite(src, dst) which will allow safe copying. And we should not even need to do checkpoint/vacuum anymore.

[wonder-sk]

Fixed in c03c33f and 348d77d