> (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.

{
    shared->macros.set(std::move(macros));
}
ContextMutablePtr Context::getQueryContext() const
{
    auto ptr = query_context.lock();
    if (!ptr) throw Exception("There is no query or query context has expired", ErrorCodes::THERE_IS_NO_QUERY);
    return ptr;
}
bool Context::isInternalSubquery() const
{
    auto ptr = query_context.lock();
    return ptr && ptr.get() != this;
}
ContextMutablePtr Context::getSessionContext() const
{
    auto ptr = session_context.lock();
    if (!ptr) throw Exception("There is no session or session context has expired", ErrorCodes::THERE_IS_NO_SESSION);
    return ptr;
}
ContextMutablePtr Context::getGlobalContext() const
{
    auto ptr = global_context.lock();
    if (!ptr) throw Exception("There is no global context or global context has expired", ErrorCodes::LOGICAL_ERROR);
    return ptr;
}
ContextMutablePtr Context::getBufferContext() const