Lazy load read receipts from the cache

[manuroe]

There will be probably a API break but we definitely to load all of the millions of RRs at startup.

We want to kill the 1035ms 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