Add merge_selecting_sleep_ms setting

Why it matters

This setting allows users to fine-tune the pause duration between merge selecting steps, which can help manage system resource usage and improve performance stability during large or intensive merges.

How to use it

Set the merge_selecting_sleep_ms parameter with an integer value representing the number of milliseconds to sleep during merge selecting. This can be configured in the server or session settings, for example:

SET merge_selecting_sleep_ms = 100;