> (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->format_schema_path = path;
}
Context::SampleBlockCache & Context::getSampleBlockCache() const
{
    assert(hasQueryContext());
    return getQueryContext()->sample_block_cache;
}
bool Context::hasQueryParameters() const
{
    return !query_parameters.empty();
}
const NameToNameMap & Context::getQueryParameters() const
{
    return query_parameters;
}
void Context::setQueryParameter(const String & name, const String & value)
{
    if (!query_parameters.emplace(name, value).second)
        throw Exception("Duplicate name " + backQuote(name) + " of query parameter", ErrorCodes::BAD_ARGUMENTS);
}
void Context::addQueryParameters(const NameToNameMap & parameters)
{
    for (const auto & [name, value] : parameters)
        query_parameters.insert_or_assign(name, value);
}
void Context::addBridgeCommand(std::unique_ptr<ShellCommand> cmd) const