cache implementation doesn't support shared tables

The table name in sql queries produced by cache.inc is dynamic. The current implementation replaces the table name after calling db_prefix_tables() which results in an incompatibility with a shared table installation of drupal if the cache table is configured not to use the default prefix in settings.php:

$db_prefix = array(
'default' => 'foo_',
'cache' => 'bar_',
)

I attached a patch that resolves this issue.