MXFileStore: Less threads switching

Each [MXFileStore saveXXX] operation (they are currently 7 save operations: saveRoomsMessages, saveRoomsState, ... ) posts a block on the file store processing queue.
That means that for one [MXFileStore commit], we can launch 7 times the file store processing queue thread. (This is actually 8 because the [MXFileStore commit] does an additional dispatch_async).

That makes a lot of thread switching which can penalise performance.

The code can be refactored to go only once on the file store processing queue.

Start by locating MXFileStore and reading the saveRoomsMessages, saveRoomsState, and other saveXXX operations together with MXFileStore commit. Trace where each operation dispatches to the file store processing queue, then verify that one commit uses a single queue dispatch while preserving the existing save behavior.

Written by the indexing model from the issue text.