planner: Optimize Plan Cache Performance

## Enhancement
Plan cache is a critical component for TiDB performance, especially in real-world transactional workloads, as it helps reduce planning cost and improve query execution efficiency.

During recent work and observation, we found that there are some unnecessary processing steps and extra overhead in the current plan cache implementation, which may be optimized to further improve performance.

This issue is intended to track possible performance optimizations for plan cache.
Further analysis and concrete optimization proposals will be added incrementally.