Set background fetch and schedule pool sizes for optimal production performance

Why it matters

The feature adjusts the default values of background_fetches_pool_size to 16 and background_schedule_pool_size to 512, optimizing ClickHouse for production workloads that involve frequent small insert operations. This change helps to better manage concurrent background tasks, leading to improved throughput and stability in such scenarios.

How to use it

Users can benefit from this feature by using the updated default configuration or manually setting the parameters in the server configuration file as follows:

<background_fetches_pool_size>16</background_fetches_pool_size>
<background_schedule_pool_size>512</background_schedule_pool_size>

No additional changes are required if running the updated ClickHouse version with these defaults.