feat: add storagechange object pool for better performance

[MatusKysel]

Description

This PR introduces an object pool for storageChange entries in the state journal to improve memory allocation performance. The changes include:

1. Added a sync.Pool for storageChange objects to reduce GC pressure
2. Modified storageChange methods to work with pointers instead of values
3. Optimized memory allocations in journal operations

Changes

• Introduced storageChangePool using sync.Pool for efficient object reuse
• Changed storageChange methods to use pointer receivers
• Optimized journal.copy() to pre-allocate slices with exact capacity
• Improved memory management in storageChange operations

Performance Impact

The changes reduce memory allocations by:

• Reusing storageChange objects instead of creating new ones
• Reducing GC pressure through object pooling

Benchmark

Old profile (note % not total GB as I was running it for different blocks)

New Profile after the change

[Copilot]

Pull Request Overview

This PR adds an object pool for storageChange entries to cut down on allocations and GC pressure when journalling state changes.

• Introduces sync.Pool for reusable storageChange instances
• Converts storageChange methods to pointer receivers and updates journal operations to track and reset pooled entries
• Optimizes journal.copy() and slice pre-allocation

[Copilot]

Pull Request Overview

This PR improves memory allocation performance in the state journal by reusing storageChange objects via a sync.Pool.

• Introduces a storageChangePool and updates storageChange methods to pointer receivers
• Tracks pooled storageChange objects in journal.storageEntries and returns them in journal.reset
• Optimizes journal.copy to pre-allocate slices and clone existing data

Comments suppressed due to low confidence (1)

core/state/journal.go:63

• Add unit tests for the pooling behavior (e.g., verify that reset returns objects to the pool and that subsequent Get calls reuse instances) to catch regressions in memory reuse.

var storageChangePool = sync.Pool{