Remove a dead store in Piece
Stuff memory holes

Padding changes on x86_64:
- RequestGroup: 29 -> 5 bytes
- DownloadContext:  25 -> 1 bytes
- Cookie: 20 -> 4 bytes
- Command: 8 -> 0 bytes (affects many derived as well)
- HttpRequest: 20 -> 4 bytes
- DownloadCommand: 18 -> 2 bytes
- TimeBasedCommand: 14 -> 6 bytes
- AbstractCommand: 13 -> 5 bytes
- HttpSkipResponseCommand: 12 -> 4 bytes
- FileEntry: 10 -> 2 bytes
- DownloadResult: 15 -> 7 bytes
- UriStruct: 12 -> 4 bytes
- Piece: 11 -> 3 bytes
- BitfieldMan -> 11 -> 3 bytes