fix: race condition

[colinkim]

refactor: fix data races in consensus core and timer management

• Add timerMu (sync.Mutex) to synchronize timer stop/set operations.
• Implement snapshotting for current Sequence and Round in timers to prevent races during view changes.
• Ensure thread-safety for futurePreprepareTimer in handlePreprepareMsg.
• Prevent potential deadlocks and nil pointer dereferences in callback routines.

These changes improve the stability of the consensus engine.

Close #41