Problem/Motivation
Drupal\Core\Cache\MemoryCache\MemoryCache is a viable cache backend that can be used as a static cache replacement on its own, or in combination with BackendChain. See #2412669: Remove drupal_static from BookManager for an example.
Drupal\Core\Cache\MemoryBackend is a testing-only implementation of a cache backend, designed to mimic APC or database caching using PHP memory (i.e. it serializes and unserializes).
MemoryBackend should probably move under a testing namespace somewhere (with the current class deprecated), and MemoryCache should be refactored to not extend from it. Potentially we could even reverse the inheritance so that the testing backend inherits from MemoryCache.
Remaining tasks
The installer users MemoryBackend - likely because MemoryCache wasn't available at the time. However changing this should probably happen in isolation in a spin-off issue since it will change behaviour between serialize/unserialize.
#3223580: Use MemoryCache (not MemoryBackend) in the installer