Support DB prefix in sql sanitize

Support for #254

To use:
Put brackets in tables of hook_sql_sync_sanitize() queries.
```drush --db-prefix sqlsan```

hook_sql_sync_sanitize() implementations that use table brackets will work just fine without the --db-prefix flag if the site is not using table prefixes.
Don't think this is needed as the _furtner() line above also tries to bootstrap more.