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/util/signature.cr

@@ -54,9 +54,11 @@ class Dir
   # Rescan conditions:
   #   - When a file added, moved, removed, renamed (including which in nested
   #       directories)
-  def self.contents_signature(dirname, cache = {} of String => String) : String
-    return cache[dirname] if !cache.nil? && cache[dirname]?
-    Fiber.yield # Yield first
+  def self.contents_signature(dirname,
+                              cache = {} of String => String,
+                              counter : YieldCounter? = nil) : String
+    return cache[dirname] if cache[dirname]?
+    counter.count_and_yield unless counter.nil?
     signatures = [] of String
     self.open dirname do |dir|
       dir.entries.sort.each do |fn|
@@ -69,10 +71,11 @@ class Dir
           #   supported file
           signatures << fn if is_supported_file fn
         end
+        counter.count_and_yield unless counter.nil?
       end
     end
     hash = Digest::SHA1.hexdigest(signatures.join)
-    cache[dirname] = hash unless cache.nil?
+    cache[dirname] = hash
     hash
   end
 end