> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

{
    std::vector<Strings> res;
    const auto * array_of_shards = typeid_cast<const ASTFunction *>(&ast);
    if (!array_of_shards || (array_of_shards->name != "array"))
        throw Exception(
            ErrorCodes::CANNOT_PARSE_BACKUP_SETTINGS,
            "Setting cluster_host_ids has wrong format, must be array of arrays of string literals");
    if (array_of_shards->arguments)
    {
        const ASTs shards = array_of_shards->arguments->children;
        res.resize(shards.size());
        for (size_t i = 0; i != shards.size(); ++i)
        {
            const auto * array_of_replicas = typeid_cast<const ASTLiteral *>(shards[i].get());
            if (!array_of_replicas || (array_of_replicas->value.getType() != Field::Types::Array))
                throw Exception(
                    ErrorCodes::CANNOT_PARSE_BACKUP_SETTINGS,
                    "Setting cluster_host_ids has wrong format, must be array of arrays of string literals");
            const auto & replicas = array_of_replicas->value.get<const Array &>();
            res[i].resize(replicas.size());
            for (size_t j = 0; j != replicas.size(); ++j)
            {
                const auto & replica = replicas[j];
                if (replica.getType() != Field::Types::String)
                    throw Exception(