must not fail if MemCache class isn't available

It can occur during a lot of situation:
- enabled in FPM but not in CLI (so `wp-cli` fails): I guess it's shouldn't even load if `PHP_SAPI === 'cli'`
- during a PHP socket switch / version upgrade
- during a debugging attempt that imply deactivating so PHP modules
- ...

In all these and others cases, the extensions should definitely NOT trigger a fatal error.

Start by locating where the MemCache class or extension is loaded, with particular attention to PHP_SAPI === 'cli' and the object-cache bootstrap entry point. Reproduce the setup with the Memcached extension unavailable in CLI, during a version switch, or after module deactivation; done means no fatal error occurs in those cases.

Written by the indexing model from the issue text.