> (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.

Context::Context(const Context &) = default;
Context & Context::operator=(const Context &) = default;
SharedContextHolder::SharedContextHolder(SharedContextHolder &&) noexcept = default;
SharedContextHolder & SharedContextHolder::operator=(SharedContextHolder &&) noexcept = default;
SharedContextHolder::SharedContextHolder() = default;
SharedContextHolder::~SharedContextHolder() = default;
SharedContextHolder::SharedContextHolder(std::unique_ptr<ContextSharedPart> shared_context)
    : shared(std::move(shared_context)) {}
void SharedContextHolder::reset() { shared.reset(); }
ContextMutablePtr Context::createGlobal(ContextSharedPart * shared)
{
    auto res = std::shared_ptr<Context>(new Context);
    res->shared = shared;
    return res;
}
void Context::initGlobal()
{
    assert(!global_context_instance);
    global_context_instance = shared_from_this();
    DatabaseCatalog::init(shared_from_this());
    EventNotifier::init();
}
SharedContextHolder Context::createShared()
{
    return SharedContextHolder(std::make_unique<ContextSharedPart>());
}
ContextMutablePtr Context::createCopy(const ContextPtr & other)