Add MergeStart and MutateStart Events to system.part_log for Merges Analysis

Why it matters

This feature provides enhanced visibility into the internal merge and mutate processes by logging their start events. It helps users analyze and visualize merges and mutations, improving monitoring and debugging capabilities for storage parts management.

How to use it

Users can query the system.part_log table to see the new MergeStart and MutateStart event entries. No additional configuration is required; simply perform merges or mutations and inspect the logs for these events.