Use JSON instead of Yajl to improve performance of store/load files (#4759)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 
When I read 10 GB file using in_tail plugin, it calls Yajl methods many
times when it stores to buffer file.

Recently, Ruby's JSON has huge improvement and it is much faster.
Ref.
https://byroot.github.io/ruby/json/2024/12/15/optimizing-ruby-json-part-1.html

* Before
  * It spent 88.533409329 sec to handle 10 GB file
* After
  * It spent 72.324643557 sec to handle 10 GB file


* config
```
<source>
  @type tail
  path /home/watson/prj/sandbox/fluentd/log/access*.log
  pos_file /home/watson/prj/sandbox/fluentd/log/access.log.pos
  tag log
  read_from_head true
  follow_inodes true
  # rotate_wait 0
  refresh_interval 5
  # open_on_every_update true
  <parse>
    @type none
  </parse>
</source>

<match **>
  @type file
  path /home/watson/prj/sandbox/fluentd/log/log
</match>
```

**Docs Changes**:

**Release Note**:

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

lib/fluent/plugin/formatter_out_file.rb

@@ -16,7 +16,7 @@
 
 require 'fluent/plugin/formatter'
 require 'fluent/time'
-require 'yajl'
+require 'json'
 
 module Fluent
   module Plugin
@@ -46,7 +46,7 @@ def format(tag, time, record)
         header = ''
         header << "#{@timef.format(time)}#{@delimiter}" if @output_time
         header << "#{tag}#{@delimiter}" if @output_tag
-        "#{header}#{Yajl.dump(record)}#{@newline}"
+        "#{header}#{JSON.generate(record)}#{@newline}"
       end
     end
   end

lib/fluent/plugin/storage_local.rb

@@ -19,7 +19,7 @@
 require 'fluent/plugin/storage'
 
 require 'fileutils'
-require 'yajl'
+require 'json'
 
 module Fluent
   module Plugin
@@ -90,7 +90,7 @@ def configure(conf)
                 log.warn "detect empty plugin storage file during startup. Ignored: #{@path}"
                 return
               end
-              data = Yajl::Parser.parse(data)
+              data = JSON.parse(data)
               raise Fluent::ConfigError, "Invalid contents (not object) in plugin storage file: '#{@path}'" unless data.is_a?(Hash)
             rescue => e
               log.error "failed to read data from plugin storage file", path: @path, error: e
@@ -114,7 +114,7 @@ def load
         return unless File.exist?(@path)
         begin
           json_string = File.open(@path, 'r:utf-8'){ |io| io.read }
-          json = Yajl::Parser.parse(json_string)
+          json = JSON.parse(json_string)
           unless json.is_a?(Hash)
             log.error "broken content for plugin storage (Hash required: ignored)", type: json.class
             log.debug "broken content", content: json_string
@@ -130,7 +130,11 @@ def save
         return if @on_memory
         tmp_path = @path + '.tmp.' + Fluent::UniqueId.hex(Fluent::UniqueId.generate)
         begin
-          json_string = Yajl::Encoder.encode(@store, pretty: @pretty_print)
+          if @pretty_print
+            json_string = JSON.pretty_generate(@store)
+          else
+            json_string = JSON.generate(@store)
+          end
           File.open(tmp_path, 'w:utf-8', @mode) { |io| io.write json_string; io.fsync }
           File.rename(tmp_path, @path)
         rescue => e