Background Merge Scheduling Improvements in ClickHouse

Background merges can be preempted by each other and they are scheduled with appropriate priorities. Now long running merges won't prevent short merges to proceed. This is needed for a better scheduling and controlling of merges execution. It reduces the chances to get "too many parts" error. #22381. #25165 (Nikita Mikhaylov). Added an ability to execute more merges and mutations than the number of threads in background pool. Merges and mutations will be executed step by step according to their sizes (lower is more prioritized). The ratio of the number of tasks to threads to execute is controlled by a setting background_merges_mutations_concurrency_ratio, 2 by default. #29140 (Nikita Mikhaylov).