Concurrent Merge Scheduling Policy Updated to Round-Robin by Default

Concurrent merges are scheduled using round-robin by default to ensure fair and starvation-free operation. Previously in heavily overloaded shards, big merges could possibly be starved by smaller merges due to the use of strict priority scheduling. Added background_merges_mutations_scheduling_policy server config option to select scheduling algorithm (round_robin or shortest_task_first). #46247 (Sergei Trifonov).