Evaluate better storage than in-memory map

[danvratil]

With many channels and active team, the in-memory cache will grow very quickly. An alternative storage, like SQLite could be used in this case, or the cache must be made smarter about what it really caches.

[danvratil]

SQLite doesn't make sense for fast querying and live display.

Instead, a tree-like structure, maybe built on top of QAIM, could make this much simpler. The model can dynamically fetch missing contents. The model can internally expire old contents. It would also make it simpler to represent the contents in QML.

Rather than simple QVariantMap, we can have a proper node tree. Channels and messages can be represented as Q_GADGETs, which are fairly cheap thanks to static metaobject. The question is whether Qt in SFOS is new enough to have proper gadget support in QML :-)

After the failed attempt with reactions, this is now a pre-requisite for any further work.

The important part is fast enough lookup for messages. The message ID is a timestamp and are sorted by the timestamp, so we should be able to have a fast lookup with binary search. We just need a representation other than QString, that doesn't compare well :-)