> (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.

M(Bool, allow_non_empty_tables) \
    M(RestoreAccessCreationMode, create_access) \
    M(Bool, allow_unresolved_access_dependencies) \
    M(RestoreUDFCreationMode, create_function) \
    M(Bool, internal) \
    M(String, host_id) \
    M(String, coordination_zk_path)
RestoreSettings RestoreSettings::fromRestoreQuery(const ASTBackupQuery & query)
{
    RestoreSettings res;
    if (query.settings)
    {
        const auto & settings = query.settings->as<const ASTSetQuery &>().changes;
        for (const auto & setting : settings)
        {
#define GET_SETTINGS_FROM_RESTORE_QUERY_HELPER(TYPE, NAME) \
            if (setting.name == #NAME) \
                res.NAME = SettingField##TYPE{setting.value}.value; \
            else
            LIST_OF_RESTORE_SETTINGS(GET_SETTINGS_FROM_RESTORE_QUERY_HELPER)
            throw Exception(ErrorCodes::CANNOT_PARSE_BACKUP_SETTINGS, "Unknown setting {}", setting.name);
        }
    }
    if (query.base_backup_name)
        res.base_backup_info = BackupInfo::fromAST(*query.base_backup_name);
    if (query.cluster_host_ids)
        res.cluster_host_ids = BackupSettings::Util::clusterHostIDsFromAST(*query.cluster_host_ids);
    return res;
}
void RestoreSettings::copySettingsToQuery(ASTBackupQuery & query) const