Add config 'forcely_yield_count'

the default value 1000 would make a fiber yield on each 4ms on SSD

Apply yield counter in Dir.contents_signauture
Use contents_signature cache in Title.new

src/library/library.cr

@@ -133,8 +133,9 @@ class Library
 
     storage = Storage.new auto_close: false
 
+    count = Config.current.forcely_yield_count
     examine_context : ExamineContext = {
-      file_counter:              (YieldCounter.new 1000),
+      file_counter:              (YieldCounter.new count),
       cached_contents_signature: {} of String => String,
       deleted_title_ids:         [] of String,
       deleted_entry_ids:         [] of String,