Removed the old argcache system so that the new one is used by default (i.e.
ENABLE_NEW_ARGCACHE is no longer necessary).
Upgrade note 1: There is no compatibility fallback to read argcache entries
from the old database. An upgrade instead depends on that the image cache
remains intact for long enough so that old images can be served without
requiring their argcache entries.
Upgrade note 2: The old 'arguments' table in the local database is not
dropped automatically. Administrators are adviced to do that to free up