Limit for Background Fetch Connections Increased and Settings Updated

The limit for the number of connections per endpoint for background fetches was raised from 15 to the value of background_fetches_pool_size setting. - MergeTree-level setting replicated_max_parallel_fetches_for_host became obsolete - MergeTree-level settings replicated_fetches_http_connection_timeout, replicated_fetches_http_send_timeout and replicated_fetches_http_receive_timeout are moved to the Server-level. - Setting keep_alive_timeout is added to the list of Server-level settings. #57523 (Nikita Mikhaylov).