> (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->system_logs)
        return {};
    return shared->system_logs->cache_log;
}
std::shared_ptr<AsynchronousInsertLog> Context::getAsynchronousInsertLog() const
{
    auto lock = getLock();
    if (!shared->system_logs)
        return {};
    return shared->system_logs->asynchronous_insert_log;
}
CompressionCodecPtr Context::chooseCompressionCodec(size_t part_size, double part_size_ratio) const
{
    auto lock = getLock();
    if (!shared->compression_codec_selector)
    {
        constexpr auto config_name = "compression";
        const auto & config = getConfigRef();
        if (config.has(config_name))
            shared->compression_codec_selector = std::make_unique<CompressionCodecSelector>(config, "compression");
        else
            shared->compression_codec_selector = std::make_unique<CompressionCodecSelector>();
    }
    return shared->compression_codec_selector->choose(part_size, part_size_ratio);
}
DiskPtr Context::getDisk(const String & name) const