> (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.

{
    if (auto elem = getProcessListElement())
        elem->cancelQuery(true);
}
String Context::getDefaultFormat() const
{
    return default_format.empty() ? "TabSeparated" : default_format;
}
void Context::setDefaultFormat(const String & name)
{
    default_format = name;
}
String Context::getInsertFormat() const
{
    return insert_format;
}
void Context::setInsertFormat(const String & name)
{
    insert_format = name;
}
MultiVersion<Macros>::Version Context::getMacros() const
{
    return shared->macros.get();
}
void Context::setMacros(std::unique_ptr<Macros> && macros)