Make the metadata cache time a static so it can be overruled.

Especially useful in situations where you not only make the current version of a plugin available, but also older (versioned) versions. For the older versions, you can presume that the meta data will not change anymore and can choose to use a longer cache time.

The reason for using a static here is of course that the method doing the setting of the cache is static as well and therefore doesn't have access to non-static class properties.