out_copy does not actually copy records

out_copy plugin is expected to send exactly same records to each sub plugins.
When a plugin which modifies records like record_modifier plugin is used, the subsequent plugins get records modified by previous plugins.

This patch fixes this issue by deeply copying records(EventStream).

----
In a following simple configration,
```
<match test>
  type copy
  <store>
    type record_modifier
    tag test.aa
    foo bar
  </store>
  <store>
    type stdout
  </store>
</match>
<match **>
  type stdout
</match>
```

When you send a message:
```
echo '{"aaa":"bbb"}' | fluent-cat test
```

You expect following results.
```
test.aa: {"aaa":"bbb","foo":"bar"}
test: {"aaa":"bbb"}
```

But you get results as blow.
```
test.aa: {"aaa":"bbb","foo":"bar"}
test: {"aaa":"bbb","foo":"bar"}
```