> (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.

{
    return shared->async_insert_queue.get();
}
void Context::setAsynchronousInsertQueue(const std::shared_ptr<AsynchronousInsertQueue> & ptr)
{
    using namespace std::chrono;
    if (std::chrono::milliseconds(settings.async_insert_busy_timeout_ms) == 0ms)
        throw Exception("Setting async_insert_busy_timeout_ms can't be zero", ErrorCodes::INVALID_SETTING_VALUE);
    shared->async_insert_queue = ptr;
}
void Context::initializeBackgroundExecutorsIfNeeded()
{
    auto lock = getLock();
    if (shared->are_background_executors_initialized)
        return;
    const auto & config = getConfigRef();
    size_t background_pool_size = 16;
    if (config.has("background_pool_size"))
        background_pool_size = config.getUInt64("background_pool_size");
    else if (config.has("profiles.default.background_pool_size"))
        background_pool_size = config.getUInt64("profiles.default.background_pool_size");
    size_t background_merges_mutations_concurrency_ratio = 2;
    if (config.has("background_merges_mutations_concurrency_ratio"))
        background_merges_mutations_concurrency_ratio = config.getUInt64("background_merges_mutations_concurrency_ratio");
    else if (config.has("profiles.default.background_merges_mutations_concurrency_ratio"))
        background_merges_mutations_concurrency_ratio = config.getUInt64("profiles.default.background_merges_mutations_concurrency_ratio");
    size_t background_move_pool_size = 8;