Do not store the 'mediaVersion' in the database

[johanjanssens]

Joomla stores a 'mediaVersion' in the database. The media version is used to force browsers to reload the CSS and JavaScript after a core update.

See : https://github.com/joomla/joomla-cms/blob/staging/libraries/cms/version/version.php#L178

Instead of storing the media version in the database generate it on the fly and cache it per request. This saves at least one database query, responsible to load the extension table, and make the core less dependant on the database.