Improve Activity log methods

[dcolleoni]

Improvements on how the logs are stored in the DB:

[andrey-katz-systemorph]

Suggestions for improvement:

1. If the import or export fail, they should not have a "fake" source Id. This should be null
2. Change the API as suggested by @dtrzesniak : WithStoreActivityLog().ExecuteAsync()
3. Detect the relevant partitions for the import

[dcolleoni]

Further improvements on the code taken from comments in PR #178

Can't you simplify this:

Add the "await DataSource.UpdateAsync(importFile.RepeatOnce());"," inside the try
Remove importSucceeded,
Add "activity = activity with {Category = "Import from File"};"," inside the try and catch --> you are creating activity twice (once to set SourceId, once to set Category).
Declare importFile inside the try
Extract the try/catch as method of type and pass the parameters to avoid the repetition of this code.
I have not thought it through, but I guess this should work and make the code leaner

the pattern matching can then be used instead of the switch case.