The cache invalidation check was previously occuring every time either
the `set` or `get` function was called on the cache. However, the cache
invalidation check is based on the jsrev which is static for the
lifetime of the page.

This change moved the invalidation to happen during the setup of the AMD
module such that it only happens one time per storage type (Local +
Session).

We only use the jsrevPrefix to determine if the cache should be
invalidated, but the prefix that we were using is based on the new
jsrev.

For example, the jsrevPrefix will be:

    hash(wwwroot + '/ + config.jsrev) + '/jsrev'

Where config.jsrev is the _current_ (new) jsrev.

As a result when searching for the jsrev used to store the data which is
currently in the storage cache, no key is returned, and we instead set
an 'initial' value and the cache is not cleared

This patch changes the jsrevPrefix to be:

    hash(wwwroot) + '/jsrev'

Since the wwwroot does not change, the key remains static for the
current site. As a result, when the jsrev is bumped via a Moodle cache
purge, we are able to correctly fetch the old jsrev from the cache,
determine that the jsrev has changed, and purge the cache.