Allow setting of global options

[tollmanz]

PECL Memcached exposes a lot of options. This library allows you to set some of them via the wp_cache_set_option() function. While this will work in some cases, it's not a great way to set options for the object cache prior to core using the cache.

I would like to extend $memcached_servers into something like $memcached_config that contains these options. These options would be initiated as soon as the object cache is initiated. This would also make it easier to test these options in Travis CI.