WL#4156: Support for connection-local storage for plugins

MySQL will provide memory where a plugin can store connection-local data

Storage engines already have it. We'll use this method for all plugins.
In THD there will be an array of pointers, one per plugin. A plugin can use it
any way it wants.

There will be a close_connection() method in the plugin structure, it'll be
called where current handlerton::close_connection() is called, but for all
plugins for which that pointer is not NULL (like we do now for storage engines).

In addition we can also have end_of_statement() callback