Fix deadlock in ResumeTransfers, mmap safety in ScheduleTransfers

[pbanakar-microsoft]

Bug fixes for resume path in azcopy engine:

• Deadlock fix: Changed ResumeTransfers to use READ lock (via buildmode.IsMover) instead of write lock when iterating jobPartMgrs to queue parts. Write lock caused deadlock when partsChannel fills up: ResumeTransfers holds write lock -> reportJobPartDoneHandler needs read lock -> scheduleJobParts blocks on unbuffered jobPartProgress.
• Mmap safety: Cached isFinalPart and cachedNumTransfers before the transfer loop in ScheduleTransfers to avoid accessing memory-mapped plan data that may be unmapped during iteration.