> (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.

ReplicatedFetchList & Context::getReplicatedFetchList() { return shared->replicated_fetch_list; }
const ReplicatedFetchList & Context::getReplicatedFetchList() const { return shared->replicated_fetch_list; }
String Context::resolveDatabase(const String & database_name) const
{
    String res = database_name.empty() ? getCurrentDatabase() : database_name;
    if (res.empty())
        throw Exception("Default database is not selected", ErrorCodes::UNKNOWN_DATABASE);
    return res;
}
String Context::getPath() const
{
    auto lock = getLock();
    return shared->path;
}
String Context::getFlagsPath() const
{
    auto lock = getLock();
    return shared->flags_path;
}
String Context::getUserFilesPath() const
{
    auto lock = getLock();
    return shared->user_files_path;
}
String Context::getDictionariesLibPath() const
{
    auto lock = getLock();
    return shared->dictionaries_lib_path;
}
String Context::getUserScriptsPath() const