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/config.cr

@@ -23,6 +23,7 @@ class Config
   property cache_enabled = false
   property cache_size_mbs = 50
   property cache_log_enabled = true
+  property forcely_yield_count = 1000
   property disable_login = false
   property default_username = ""
   property auth_proxy_header_name = ""