memtx: introduce read view statistics

Closes #8501

@TarantoolBot document
Title: Document memtx read view statistics

New entries have been added to `box.stat.memtx()` output:

```
tarantool> box.stat.memtx().data
---
- garbage: 0
  total: 24986
  read_view: 0
...

tarantool> box.stat.memtx().index
---
- read_view: 0
  total: 933888
...
```

`data` shows how much memory is allocated for memtx tuples:
 - `data.total` - total amount of memory allocated for data tuples.
   This includes `data.read_view` and `data.garbage` plus tuples that
   are actually stored in memtx spaces.
 - `data.read_view` - memory held for read views.
 - `data.garbage` - memory that is unused and scheduled to be freed
   (freed lazily on memory allocation).

`index` shows how much memory is allocated for memtx index extents:
 - `index.total` - total amount of memory allocated for indexing data.
   This includes `index.read_view` plus memory used for indexing tuples
   that are actually stored in memtx spaces.
 - `index.read_view` - memory held for read views.

All numbers are given in bytes.

`data.read_view` and `index.read_view` include memory allocated both for
system read views (snapshot, replication) and user read views (EE-only).
They should be non-zero only if there are open read views. To list all
open read views, use `box.read_view.list()`.