plugin: fix a crash bug on multiple DB open case

There are the following two conditions for the crash bug:

  * A plugin has two or more normalizers/tokenizers
    (e.g.: groonga-normalizer-mysql) is used.
  * The same database is opened from two or more in a process
    (mroonga use case).

The first database closing closes the plugin even if the rest opened
database uses the plugin.

[groonga-dev,01596]

Reported by Naoya Murakami. Thanks!!!