box: implement box.read_view.list

To support read view listing, we need to add name, id, system flag,
timestamp, vclock, and signature to struct read_view. (Previously they
were stored in Lua read view object implemented in Tarantool EE.)
Also, we have to maintain a registry of all active read views in C.
The registry pursues two goals:

 1. It's used for pushing read view objects (which may be created
    entirely in C, circumventing Lua code) to Lua.
 2. We look up a read view in the registry by id to query the read view
    status ('open' or 'closed') from Lua. This is required so that a
    read view object returned by box.read_view.list() and cached by the
    caller reports the up-to-date status. If a read view isn't found in
    the registry, then it must be closed.

Apart from the C registry of active read views, we also maintain a Lua
registry of all read views that are used in Lua. We add read view
objects returned by box.read_view.list() to this registry so that the
next call would return the same objects. The Lua registry is backed by
a weak table so that it doesn't pin a closed read view object when the
caller drops the last reference to it.

We also intend to move all read view listing machinery from the EE code
to CE (Lua registry, metatables). The EE code will need to override two
methods box.read_view.open() and box.internal.read_view_close(), which
are stubbed out in the CE code. To set the metatable for a read view
object and add it to the registry, EE version of box.read_view.open()
will use box.internal.read_view_register().

Closes #8260

@TarantoolBot document
Title: Document box.read_view.list

The new function returns an array of all active database read views.
It includes both read views created for system purposes (e.g. to make a
checkpoint or join a new replica) and read views created by application
code (this feature is available only in Tarantool Enterprise Edition,
see https://github.com/tarantool/enterprise_doc/issues/194).

Each read view is represented by a table with the following fields:
 - `id` - unique read view identifier.
 - `name` - read view name.
 - `is_system` - true if the read view is used for system purposes.
 - `timestamp` - `fiber.clock()` when the read view was opened.
 - `vclock` - `box.info.vclock` when the read view was opened.
 - `signature` - `box.info.signature` when the read view was opened.
 - `status` - 'open' or 'closed'.

Read views created by application code also have the 'space' field,
which lists all spaces available in the read view, and may be used just
like a read view object returned by `box.read_view.open()`.

The array is sorted by read view id. Since read view ids grow
monotonically, this means that the most recent read view goes last.

Example:

```
tarantool> box.read_view.list()
---
- - timestamp: 4057333.4969064
    signature: 3
    is_system: true
    status: open
    vclock: {1: 3}
    name: join
    id: 3
  - timestamp: 4057357.0874192
    signature: 6
    is_system: true
    status: open
    vclock: {1: 6}
    name: checkpoint
    id: 4
...
```