Make the version of Spider the same as the server version

Once upon a time, Spider was an external storage engine, and thus it was reasonable to have its own version number apart from MariaDB's one. However, now Spider is a part of the server. Change the Spider's version to be the server version, to avoid manually changing it at every release.

Once the Spider version comes to be same as the one of the server, the variable spider_version is no longer necessary. Delete it.

We also need to update docs (maybe by a separated issue).