roots: add 'recent' crawl mode for a mix of new and old data

Crawl mode 3 'recent' prioritizes data from new updates to previously
scanned subvols over subvols that have not been completely scanned yet.
If no such new data exists, falls back to a variation of 'lockstep'
scan mode.

This enables us to keep up with new data as it arrives, a key weakness
of all the other scan modes, and worth violating our unwritten "no new
scan modes until we have extent-tree dedupe working" policy for.

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

src/bees.h

@@ -596,6 +596,7 @@ public:
 		SCAN_MODE_LOCKSTEP,
 		SCAN_MODE_INDEPENDENT,
 		SCAN_MODE_SEQUENTIAL,
+		SCAN_MODE_RECENT,
 		SCAN_MODE_COUNT, // must be last
 	};