Make code-dependent cache entries more volatile

Make code-dependent cache entries more volatile

If a cache entry is dependent on the code that's running, upgrading
(or enabling/disabling plugins) can generate hard-to-track
inconsistencies.

This change adds a close-to-unique fingerprint of the running code to
some cache keys, so that if the fingerprint changes, the old values
are ignored and new values are used.

If the automated uniqueness fails, an administrator can add an extra
config value, $config['site']['build'], that's thrown into the key also.