Fixed the Possibility of Hanging Queries When Server Is Overloaded and Global Thread Pool Becomes Near Full

Fixed the possibility of hanging queries when server is overloaded and global thread pool becomes near full. This have higher chance to happen on clusters with large number of shards (hundreds), because distributed queries allocate a thread per connection to each shard. For example, this issue may reproduce if a cluster of 330 shards is processing 30 concurrent distributed queries. This issue affects all versions starting from 19.2. #6301 (alexey-milovidov)