> (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.

{
    auto lock = getLock();
    shared->bridge_commands.emplace_back(std::move(cmd));
}
IHostContextPtr & Context::getHostContext()
{
    return host_context;
}
const IHostContextPtr & Context::getHostContext() const
{
    return host_context;
}
std::shared_ptr<ActionLocksManager> Context::getActionLocksManager() const
{
    auto lock = getLock();
    if (!shared->action_locks_manager)
        shared->action_locks_manager = std::make_shared<ActionLocksManager>(shared_from_this());
    return shared->action_locks_manager;
}
void Context::setExternalTablesInitializer(ExternalTablesInitializer && initializer)
{
    if (external_tables_initializer_callback)
        throw Exception("External tables initializer is already set", ErrorCodes::LOGICAL_ERROR);
    external_tables_initializer_callback = std::move(initializer);
}
void Context::initializeExternalTablesIfSet()