roots: split constructor into separate start method

This allows us to use the fd cache and inode resolve functions
without starting crawler threads.

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

src/bees-context.cc

@@ -987,7 +987,7 @@ BeesContext::start()
 	hash_table();
 
 	// Kick off the crawlers
-	roots();
+	roots()->start();
 }
 
 void

src/bees-roots.cc

@@ -546,12 +546,15 @@ BeesRoots::BeesRoots(shared_ptr<BeesContext> ctx) :
 	m_crawl_thread("crawl_transid"),
 	m_writeback_thread("crawl_writeback")
 {
-
 	m_root_ro_cache.func([&](uint64_t root) -> bool {
 		return is_root_ro_nocache(root);
 	});
 	m_root_ro_cache.max_size(BEES_ROOT_FD_CACHE_SIZE);
+}
 
+void
+BeesRoots::start()
+{
 	m_crawl_thread.exec([&]() {
 		// Measure current transid before creating any crawlers
 		catch_all([&]() {