Removing more references to Memcache
Implement `increment()` and `decrement()` methods for the corresponding `wp_cache_incr()` and `wp_cache_decr()` functions.
Remove `wp_cache_increment()` and `wp_cache_decrement()` as they aren't part of Core's implementation.

Users switching between cache backends shouldn't have to worry about function names changing. That's not how Core's object cache is designed.
Add tracking of cache hits and misses, and the attendant function that the Debug Bar plugin uses.
Match function order to Core's template and add a function that Core has but we're missing.
Consistency in function naming and internal references
Introduce `replace()` method by repurposing existing `add()` method to be more flexible.
Add handling for the various responses Predis returns after performing an action, ensuring a boolean always results.
Add method for changing the blog-specific cache prefix when switching blogs in a multisite environment.
Only clear the current DB, not all DBs in the Redis instance, when flushing the cache.
Minor cleanup