Topic info records need to have specific order defined via mNext and mPrev
fields (next and previous records). When loading multiple files a record may be
inserted into middle of the topic but neighborhood records may not be aware of
it. Having the order it's possible to move the records within one topic.
Sort the record once after loading all content files but preserve the order for
all other operations. Use std::map to group info ids by topic to make sure the
topics order is stable. Keep order within a topic for info ids on loading new
records. Use this order later for sorting the records.
