Improve memory management : modify Article to adopt the NSDiscardableContent protocol

[barijaona]

As folders use NSCache to cache articles, it might be interesting to implement the NSDiscardableContent protocol into articles.

Implementing this will require designing which parts of an article should be kept in memory as much as possible, and which ones can be discarded from memory and how the later would be reloaded from database when needed.

[barijaona]

Database and Folder already have the current mechanism which might be a start for inspiration :

vienna-rss/Vienna/Sources/Database/Database.m

Lines 2016 to 2061 in a9997b0

	/* minimalCacheForFolder
	* Returns a minimal cache of article information for the specified folder,
	* which is enough for tasks like feed refreshes, but is not complete enough
	* for displaying articles : it lacks articles' descriptions and dates.
	*/
	-(NSArray *)minimalCacheForFolder:(NSInteger)folderId
	{
	// Prime the folder cache
	[self initFolderArray];
	__block NSInteger unread_count = 0;
	NSMutableArray * myCache = [NSMutableArray array];
	FMDatabaseQueue *queue = self.databaseQueue;
	[queue inDatabase:^(FMDatabase *db) {
	FMResultSet * results = [db executeQueryWithFormat:@"SELECT message_id, read_flag, marked_flag, deleted_flag, title, link, revised_flag, hasenclosure_flag, enclosure FROM messages WHERE folder_id=%ld", (long)folderId];
	while ([results next]) {
	NSString * guid = [results stringForColumnIndex:0];
	BOOL read_flag = [results stringForColumnIndex:1].integerValue;
	BOOL marked_flag = [results stringForColumnIndex:2].integerValue;
	BOOL deleted_flag = [results stringForColumnIndex:3].integerValue;
	NSString * title = [results stringForColumnIndex:4];
	NSString * link = [results stringForColumnIndex:5];
	BOOL revised_flag = [results stringForColumnIndex:6].integerValue;
	BOOL hasenclosure_flag = [results stringForColumnIndex:7].integerValue;
	NSString * enclosure = [results stringForColumnIndex:8];
	// Keep our own track of unread articles
	if (!read_flag) {
	++unread_count;
	}
	Article * article = [[Article alloc] initWithGuid:guid];
	[article markRead:read_flag];
	[article markFlagged:marked_flag];
	[article markRevised:revised_flag];
	[article markDeleted:deleted_flag];
	article.folderId = folderId;
	article.title = title;
	article.link = link;
	article.enclosure = enclosure;
	article.hasEnclosure = hasenclosure_flag;
	[myCache addObject:article];
	}
	[results close];
	}];