> (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.

{
    return std::shared_ptr<Context>(new Context(*other));
}
ContextMutablePtr Context::createCopy(const ContextWeakPtr & other)
{
    auto ptr = other.lock();
    if (!ptr) throw Exception("Can't copy an expired context", ErrorCodes::LOGICAL_ERROR);
    return createCopy(ptr);
}
ContextMutablePtr Context::createCopy(const ContextMutablePtr & other)
{
    return createCopy(std::const_pointer_cast<const Context>(other));
}
Context::~Context() = default;
InterserverIOHandler & Context::getInterserverIOHandler() { return shared->interserver_io_handler; }
const InterserverIOHandler & Context::getInterserverIOHandler() const { return shared->interserver_io_handler; }
std::unique_lock<std::recursive_mutex> Context::getLock() const
{
    ProfileEvents::increment(ProfileEvents::ContextLock);
    CurrentMetrics::Increment increment{CurrentMetrics::ContextLockWait};
    return std::unique_lock(shared->mutex);
}
ProcessList & Context::getProcessList() { return shared->process_list; }
const ProcessList & Context::getProcessList() const { return shared->process_list; }
OvercommitTracker * Context::getGlobalOvercommitTracker() const { return &shared->global_overcommit_tracker; }
MergeList & Context::getMergeList() { return shared->merge_list; }
const MergeList & Context::getMergeList() const { return shared->merge_list; }
MovesList & Context::getMovesList() { return shared->moves_list; }
const MovesList & Context::getMovesList() const { return shared->moves_list; }