replay: fix various synchronization and event handling issues

[deanlee]

Summary of Changes

• Thread-safe shared pointer access: Protected std::shared_ptr access using std::atomic_load and std::atomic_store to ensure thread-safe reads and writes.
• Double-free fix: Resolved a double-free issue occurring during Cabana's Slider::paintEvent.
• Seek process improvement: Simplified the seeking process and fixed a potential hang when scrubbing by shift-dragging on the chart in Cabana.
• Stream thread synchronization: Ensured the stream thread doesn’t resume until the onSegmentsMerged callback is fully finished to prevent data inconsistencies.
• Synchronized slot function: Used a mutex and condition_variable to wait for the queued slot function to finish, ensuring it stays in sync with the replay's handleSegmentMerge thread.
• Callback outside stream lock: Prevented potential deadlocks by invoking callbacks outside of the stream lock.
• Code cleanup: Corrected indentation issues in timeline.h for better readability.