feat: Enhance Arena class for thread safety and add multithreading test

[zm1060]

• Implement mutex locking in the Arena class to ensure thread-safe operations across Allocate, AllocateAligned, and related methods.
• Refactor Arena's internal logic to prevent deadlocks and ensure efficient memory allocation in a multithreaded environment.
• Add a comprehensive unit test in arena_test.cc to verify thread safety, focusing on concurrent memory allocation and deallocation.
• Update documentation in arena.h to reflect new thread safety features and provide usage guidelines for multithreaded scenarios.

This commit addresses the need for thread-safe memory allocation in high-concurrency environments, improving the robustness and reliability of the Arena class in LevelDB.