box: rename a few 'cluster's to 'replicaset's

In a few places visible to users and in iproto naming the term
"cluster" really means "replicaset". One of those places is a
part of public API - box.iproto.key.CLUSTER_UUID - which is not
yet released.

The commit renames "cluster" in those places as a preparation for
introduction of actual "cluster", like a set of replicasets. It
will start from introduction of cluster name in addition to
replicaset uuid/name.

There are places which still mention 'cluster', but their rename
would be breaking. It will be addressed in scope of a bigger
patchset.

Part of #5029

NO_CHANGELOG=Was not released

@TarantoolBot document
Title: Rename `IPROTO_CLUSTER_UUID` to `IPROTO_REPLICASET_UUID`

This is a name for one of the IProto keys. The key value doesn't
change and the protocol is still backward compatible. But better
rename it to `IPROTO_REPLICASET_UUID`, because in future
`IPROTO_CLUSTER_UUID` will most likely mean a different thing.