roots: move common code for creating crawl Tasks into a method

Duplicated code between the different scan modes has slowly been
becoming less and less trivial.  Move the code to a method and
make both scan-modes call it.

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

src/bees.h

@@ -548,6 +548,7 @@ class BeesRoots : public enable_shared_from_this<BeesRoots> {
 	uint64_t next_root(uint64_t root = 0);
 	void current_state_set(const BeesCrawlState &bcs);
 	RateEstimator& transid_re();
+	size_t crawl_batch(shared_ptr<BeesCrawl> crawl);
 
 friend class BeesFdCache;
 friend class BeesCrawl;