Optimize pool refresh & prevent redundant block validation

[Rakni1988]

This update optimizes pool performance by preventing unnecessary block refreshes and duplicate block validation.

Fixes & Improvements:

• Replaces is_synced() check with block source validation, ensuring refresh occurs only when necessary (newblock from peers or a mined block by the pool).
• Prevents double-mining on an already found block, reducing wasted miner resources.
• Avoids refresh deadlocks: Previously, synced or syncing states could occasionally block refreshes during normal operation, causing the pool to keep searching for an already found block.
• Added is_verified flag to avoid redundant block validation:
  • Previously, test_block() ran twice in integrate_blocks_with_existing_chain().
  • Now, blocks marked verified won’t be re-validated before insertion.
  • Reduces unnecessary computational overhead and improves sync speed.

Result:

• More accurate block updates
• Better miner efficiency
• No wasted hashing power
• Faster chain synchronization due to reduced duplicate block checks.