This change ensures that anyone calling tcp_cache_set() directly in their own custom code has a more reliable idea of whether this method call has failed.

This commit strictly divides the TCP cache methods to stop storing a local copy of the remote cache when a persistent cache exists elsewhere.

It also adds expirations to the local cache, in the event that the PHP request is allowed to process and hang for longer than the servers have collectively to be checked.

Even for master. No matter the tries remaining.

If TCP responsiveness is being checked and the connection to master goes down, it will exist in the cache.

Renames some protected methods to better match their intended TCP usages.

Because WordPress loads WPDB before the object cache, the persistent TCP cache implementation needs to carefully start the object cache manually.

Fixes #126.