Update cacher to use monitor mixin

plugins/commands/serve/util/cacher.rb

@@ -1,40 +1,43 @@
+require "monitor"
+
 module VagrantPlugins
   module CommandServe
     module Util
       class Cacher
+        include MonitorMixin
 
         class Entry
+          include MonitorMixin
           attr_reader :value
-          attr_reader :m
 
           def initialize(value)
+            super()
             @value = value
-            @m = Mutex.new
           end
         end
 
         def initialize
-          @m = Mutex.new
+          super()
           @registry = {}
         end
 
         def registered?(key)
-          @registry.key?(key)
+          synchronize { @registry.key?(key) }
         end
 
         def []=(key, value)
           entry = Entry.new(value)
-          @m.synchronize { @registry[key] = entry }
+          synchronize { @registry[key] = entry }
         end
 
         def [](key)
-          @m.synchronize { @registry[key]&.value }
+          synchronize { @registry[key]&.value }
         end
 
         def delete(key)
           entry = @registry[key]
           return if entry.nil?
-          entry.m.synchronize do
+          entry.synchronize do
             value = @registry[key].value
             @registry.delete(key)
             value
@@ -52,7 +55,7 @@ module VagrantPlugins
               "No value cached with key `#{key}'"
           end
 
-          entry.m.synchronize do
+          entry.synchronize do
             yield entry.value
           end
         end