Split alloptions into separate cache keys

[danielbachhuber]

Doing so helps to mitigate the classic race condition described in https://core.trac.wordpress.org/ticket/31245

Inspiration from https://github.com/humanmade/memcache-object-cache/blob/master/object-cache.php#L153-L231 and https://github.com/humanmade/wordpress-pecl-memcached-object-cache/pull/1/files

[joehoyle]

@danielbachhuber sorry I missed your message yesterday, but as you discovered, this is how we fixed this issue, not the prettiest but does work yell!

[danielbachhuber]

@joehoyle Yep, turned out to be more straightforward than I expected it to be. Thanks for paving the way!