> (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.

{
    assert(metadata_transaction);
    assert(hasQueryContext());
    metadata_transaction = nullptr;
}
void Context::checkTransactionsAreAllowed(bool explicit_tcl_query ) const
{
    if (getConfigRef().getInt("allow_experimental_transactions", 0))
        return;
    if (explicit_tcl_query)
        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Transactions are not supported");
    throw Exception(ErrorCodes::LOGICAL_ERROR, "Experimental support for transactions is disabled, "
                    "however, some query or background task tried to access TransactionLog. "
                    "If you have not enabled this feature explicitly, then it's a bug.");
}
void Context::initCurrentTransaction(MergeTreeTransactionPtr txn)
{
    merge_tree_transaction_holder = MergeTreeTransactionHolder(txn, false, this);
    setCurrentTransaction(std::move(txn));
}
void Context::setCurrentTransaction(MergeTreeTransactionPtr txn)
{
    assert(!merge_tree_transaction || !txn);
    assert(this == session_context.lock().get() || this == query_context.lock().get());
    merge_tree_transaction = std::move(txn);
    if (!merge_tree_transaction)
        merge_tree_transaction_holder = {};
}
MergeTreeTransactionPtr Context::getCurrentTransaction() const