Elegant creation of schema for Watch

[darashi]

This may be solved by creating a hook in Partition, @kou said.

      # XXX just workaround. This must be re-written.
      # When secondary and later processes opens the database,
      # creation processes of tables by the first process is
      # not finished yet. Then secondary and others tries to
      # create tables and raises errors. To avoid such a problem,
      # the creation processes of tables is disabled on workers.

https://github.com/droonga/fluent-plugin-droonga/blob/master/lib/droonga/plugin/handler/watch.rb#L30-L43