replication: introduce cluster name

The patch adds 2 new entities to replication: the concept of a
cluster which has multiple replicasets and a name for this
cluster.

The name so far doesn't participate in any replication protocols.
It is just stored in _schema and is validated against the config.

The old mentions of 'cluster' (in logs, in some protocol keys like
in the feedback daemon) everywhere are now considered obsolete and
probably will be eventually replaced with 'replicaset'.

Part of #5029

@TarantoolBot document
Title: `box.cfg.cluster_name` and `box.info.cluster.name`
The new option `box.cfg.cluster_name` allows to assign the cluster
name to a human-readable text value to be displayed in the new
info key - `box.info.cluster.name` - and to be validated when the
instances in the cluster connect to each other.

The name is broadcasted in "box.id" built-in event as
"cluster_name" key. It is string when set and nil when not set.

When set, it has to match in all instances of the entire cluster
in all its replicasets.

If a name wasn't set on cluster bootstrap (was forgotten or the
cluster is upgraded from a version < 3.0), then it can be set
on an already running instance via `box.cfg.cluster_name`.

To change or drop an already installed name one has to use
`box.cfg.force_recovery == true` in all instances of the cluster.
After the name is updated and all the instances synced, the
`force_recovery` can be set back to `false`.

The name can be <= 63 symbols long, can consist only of chars
'0'-'9', '-' and 'a'-'z'. It must start with a letter. When
upper-case letters are used in `box.cfg`, they are automatically
converted to lower-case. The names are host- and DNS-friendly.