Added clearing of cache groups

[pbearne]

WordPress/performance#38
https://core.trac.wordpress.org/ticket/4476
https://core.trac.wordpress.org/ticket/9773
solves #INITS-115

Trac ticket: https://core.trac.wordpress.org/ticket/4476
https://core.trac.wordpress.org/ticket/9773)

[sboisvert]

@pbearne I worry about the flush cache group automatically flushing everything. I like the approach here: pantheon-systems/wp-redis@db32e94 that only flushes if it's possible to target them. It feels like a safer approach.

What we could do is, separately from the changes to core. Add support for groups to object cache libraries. The redis one above already implements it but I don't think any memcache ones do. I suspect it's because memcache has no concept of groups and therefore it's just a hash of the concatenation of the key and group most of the time, but we could pseudo implement it by having groups be a pointer to an incrementor and when you flush the cache group you're in essence just incrementing the group part of the cache key, which while not actually flushing the keys, would make them return false / repopulate them.

[pbearne]

@sboisvert I have removed the failback
I have refactored the functions to make them more SOLID
I will need to add some more tests for the new functions but I through I get the code/approach sorted first
Should a change the method to detect and function names to redist example?

[spacedmonkey]

You might want to revert these commits and just merge trunk into this branch. I am seeing a lot of changes that are unrelated to this PR.

[spacedmonkey]

This patch looks solid now. Let's submit it to WP Core team and get feedback from the wider community.