> (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.

{
    auto lock = getLock();
    if (!shared->user_defined_sql_objects_loader)
        shared->user_defined_sql_objects_loader = createUserDefinedSQLObjectsLoader(getGlobalContext());
    return *shared->user_defined_sql_objects_loader;
}
#if USE_NLP
SynonymsExtensions & Context::getSynonymsExtensions() const
{
    auto lock = getLock();
    if (!shared->synonyms_extensions)
        shared->synonyms_extensions.emplace(getConfigRef());
    return *shared->synonyms_extensions;
}
Lemmatizers & Context::getLemmatizers() const
{
    auto lock = getLock();
    if (!shared->lemmatizers)
        shared->lemmatizers.emplace(getConfigRef());
    return *shared->lemmatizers;
}
#endif
BackupsWorker & Context::getBackupsWorker() const
{
    auto lock = getLock();
    if (!shared->backups_worker)
        shared->backups_worker.emplace(getSettingsRef().backup_threads, getSettingsRef().restore_threads);
    return *shared->backups_worker;
}
void Context::setProgressCallback(ProgressCallback callback)