Lazy load room messages from the cache

[manuroe]

As we did in the SDK for room states when we introduced lazy loaded, we should be able to load messages from the store only when needed, ie:

• when the user opens the room and paginates in the timeline
• when we are getting more messages from /sync and we need to store them.

As everything should be already asynchronous, there should not be any api break.

We want to kill the 828ms in this log:

[MXFileStore] Start data loading from files
[MXFileStore] Loaded room messages of 625 rooms in 828ms
[MXFileStore] Loaded rooms summaries data of 625 rooms in 572ms
[MXFileStore] Loaded rooms account data of 625 rooms in 359ms
[MXFileStore] Loaded read receipts of 625 rooms in 1035ms
[MXFileStore] Loaded 2164 MXUsers in 71ms
[MXFileStore] Loaded 2 MXGroups in 1ms
[MXBaseProfiler] Task startup - storePreload for 625 units completed in 2870.760ms
[MXFileStore] Data loaded from files in 2871ms

Note this time is continuously growing with the amount of messages received and cached.