ItemMetadataDBManager improvements

[phil1995]

In detail, two optimizations were made:

• cacheMetadata(itemMetadataList:) now uses a UPSERT instead of a manual fetch and subsequent match.

• flagAllItemsAsMaybeOutdated(withParentID:)now updates the entire request at once instead of iterating through each individual item.

Both optimizations take advantage of SQLite's standard features instead of using more inefficient "workarounds" in Swift.
Furthermore, both optimizations lead to a smaller memory footprint and are thus relevant for #232.

[tobihagemann]

I wonder if the first optimization leads to significantly worse maintainability. Is the performance gain here really relevant? What do you think?

[phil1995]

I wonder if the first optimization leads to significantly worse maintainability. Is the performance gain here really relevant? What do you think?

I can understand the concern about maintainability.
However, I would argue that it is worth it in this case. On one hand, we already had to make sure that certain properties remain persisted during an update. On the other hand, we get at least a compiler error as soon as a column is no longer present, since these are referenced via ItemMetadata.Columns.columnName.
Also, this function is used on every successful call to enumerateItems(for:startingAt:) and especially for folders with many items, we should shorten the loading time as much as possible for the user. For example, for completely new folders, this halves the number of requests executed against the database.