MongoDB: fix cursor destructors

[WebFreak001]

unsure how this hasn't triggered before - checked and fixed on MongoDB 7 now

[s-ludwig]

Is there any test case that we can add?

[WebFreak001]

I can try to create a test case for this - it didn't happen for everything, I just had this start to happen on some data and I'm not sure why. Might have been because I have really large collections where I had multiple cursors doing things

[WebFreak001]

I can't manage to make a failing reproduction test case, but on my production system without this patch the program fails with:

vibe.db.mongo.connection.MongoDriverException@../.dub/packages/vibe-d-0.9.8-beta.3/vibe-d/mongodb/vibe/db/mongo/connection.d(462): command failed: Invalid namespace specified 'user_admin_todo_tbl_08D8C8DCF2B3FD74.' in vibe.db.mongo.connection.MongoConnection.killCursors (../.dub/packages/vibe-d-0.9.8-beta.3/vibe-d/mongodb/vibe/db/mongo/connection.d:636

(where user_admin_todo_tbl_08D8C8DCF2B3FD74 is my table's name)

in code:

	MongoCollection tableCollection()
	{
		return _someExistingCollection.database[name];
	}

	TableRow[] findTable()(Bson[string] query, auto ref in User user, size_t maxRows = size_t.max)
	{
		bool[] includedColumns = generateIncludedColumns(user);
		TableRow[] fields;
		auto cursor = tableCollection.find(query); // <-- caused by this destructor
		if (cursor.empty)
			return [];
		UserRowMatcher rowMatcher = getMatcher(user);
		foreach (Bson bsonRow; cursor)
		{
			...
		}
		return fields;
	}

I tried to test in a separate function:

• empty cursor
• 100000 inserts and then iterating over all

however both worked fine / didn't cause failure (with similar code layout as seen above).

Can we just merge this as-is since the tests don't regress from this and on my production system it fixes the actual issue?