roots: add scan-mode 2 "oldest crawler first"

Add a third scan mode with alternative trade-offs.

Benefits:  Good sequential read performance.  Avoids race conditions
described in https://github.com/Zygo/bees/issues/27.  Avoids diverting
scan resources into short-lived snapshots before their long-lived
origin subvols are fully scanned.

Drawbacks:  Takes the longest time of the three implemented scan-modes
to free space in extents that are shared between snapshots.  Uses the
maximum amount of temporary space.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -564,6 +564,7 @@ public:
 	enum ScanMode {
 		SCAN_MODE_ZERO,
 		SCAN_MODE_ONE,
+		SCAN_MODE_TWO,
 		SCAN_MODE_COUNT, // must be last
 	};