Improve the /stats route

[curquiza]

From @irevoire in this issue meilisearch/meilisearch#2729 I move here since it's feature request

Currently, the stats route returns the sum of the used map size of each index without any information on the task queue.
That's pretty useless, IMO.

I would like the stats route to return;

• Per index
  • The size used by each index (without the free pages)
  • The REAL size used by each index on the disk
• The size used by the whole data.ms (including the task queue and the updates stored on disk but not yet indexed)
• The real size used by the tasks queue without the free pages + the update files + the real size used by each index