is the same in the plugin and in the mysqld.
store the size in the plugin.

To simplify access to plugin's st_mysql_plugin, copy it on open and
convert to the latest version, so that the rest of the code would
not need to care about versions.


include/plugin.h:
  plugin versioning: we cannot assume that sizeof(st_mysql_plugin)
  is the same in the plugin and in the mysqld.
  store the size in the plugin.