Still crashes upon some state cache saves

[KitsuneRal]

No description provided.

[KitsuneRal]

At least one more reason for crashing is not a regression: Connection::saveState() leaves a room for accessing a deleted invitation or forgotten room object due to a race condition introduced by calling processEvents() that may delete rooms already being iterated. Possible fixes include:

• using QPointer for the rooms map (increases the space consumed by this map almost two-fold);
• pre-saving JSON for each room, with saveState() just dumping those objects to one array (given that saveState() is called from clients, we'll have to pre-save upon each room update, rather than just before saving the cache, which is potentially expensive);
• split the cache per-room (quite a considerable change to the caching code; again, either the clients will have to diligently trigger to-be Room::saveState() per each room, or we should take this on the library side altogether - which sounds not a bad idea at all, in fact).

[KitsuneRal]

The last option is now in implementation.

[KitsuneRal]

The cache is written but not read properly.