The primary reason of this change is to keep compatibility of 1.10
release series with tarantool/stat-0.3.1, which expects that each
box.stat.net.<...> and box.stat.net().<...> item is a table.

This commit changes CONNECTIONS metric to be a table with 'current'
field, which in turn contains current number of connections.

Fixes #4039.

@TarantoolBot document
Title: box.stat.net.CONNECTIONS becomes a table

The format of box.stat.net.CONNECTIONS and box.stat.net().CONNECTIONS is
changed in order to keep all items being tables, because
tarantool/stat-0.3.1 expects them to be tables (see [1] for more
information).

Example of box.stat.net() **before** this commit:

```
tarantool> box.stat.net()
---
- SENT:
    total: 0
    rps: 0
  CONNECTIONS: 0
  RECEIVED:
    total: 0
    rps: 0
...
```

And after:

```
tarantool> box.stat.net()
---
- SENT:
    total: 0
    rps: 0
  CONNECTIONS:
    current: 0
  RECEIVED:
    total: 0
    rps: 0
...
```

Look at the comment to lbox_stat_net_call() (see the linked commit) for
meaning of total/rps/current fields.

[1]: https://github.com/tarantool/tarantool/issues/4039