> (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();
    return shared->uncompressed_cache;
}
void Context::dropUncompressedCache() const
{
    auto lock = getLock();
    if (shared->uncompressed_cache)
        shared->uncompressed_cache->reset();
}
void Context::setMarkCache(size_t cache_size_in_bytes, const String & mark_cache_policy)
{
    auto lock = getLock();
    if (shared->mark_cache)
        throw Exception("Mark cache has been already created.", ErrorCodes::LOGICAL_ERROR);
    shared->mark_cache = std::make_shared<MarkCache>(cache_size_in_bytes, mark_cache_policy);
}
MarkCachePtr Context::getMarkCache() const
{
    auto lock = getLock();
    return shared->mark_cache;
}
void Context::dropMarkCache() const
{
    auto lock = getLock();
    if (shared->mark_cache)
        shared->mark_cache->reset();
}
ThreadPool & Context::getLoadMarksThreadpool() const