Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

{
    String mutation_file_name;
    Int64 version = startMutation(commands, mutation_file_name);
    if (query_context->getSettingsRef().mutations_sync > 0)
        waitForMutation(version, mutation_file_name);
}
namespace
{
struct PartVersionWithName
{
    Int64 version;
    String name;
};
bool comparator(const PartVersionWithName & f, const PartVersionWithName & s)