Use ObjectSpace::WeakKeyMap for level_override

This allows fiber keys to be GCed and removed from the map.  Otherwise,
fibers that call `#with_level` create a memory leak if they are killed
without running their ensure blocks.

Author: nevans

lib/logger.rb

@@ -379,6 +379,10 @@ class Logger
 
   include Severity
 
+  DefaultLevelOverride =
+    defined?(ObjectSpace::WeakKeyMap) ? ObjectSpace::WeakKeyMap : Hash
+  private_constant :DefaultLevelOverride
+
   # Logging severity threshold (e.g. <tt>Logger::INFO</tt>).
   def level
     level_override[level_key] || @level
@@ -606,7 +610,7 @@ def initialize(logdev, shift_age = 0, shift_size = 1048576, level: DEBUG,
     self.datetime_format = datetime_format
     self.formatter = formatter
     @logdev = nil
-    @level_override = {}
+    @level_override = DefaultLevelOverride.new
     return unless logdev
     case logdev
     when File::NULL
@@ -789,7 +793,7 @@ def level_override
         does not call super probably
       ;;;
     end
-    @level_override ||= {}
+    @level_override ||= DefaultLevelOverride.new
   end
 
   def level_key