Merge branch '3.next'

Fix grammar in Memcached Engine comment

Merge branch 'master' into 3.next

Merge branch 'master' into 3.next

Close directory handle to prevent locking issues

Merge branch 'master' into 3.next

Add SimpleCacheEngine::add()

I forgot this method last time around. I've also added soft deprecations
for Cache::engine(). Having hard deprecations would require more
refactoring than I want to do right now.

Fix typo in comment

"Determine if" not "Determine is"

Fix typo in comment

"Determine if" not "Determine is"

Add missing versions to deprecation notices.