Not found fixes

### Problem
After trying to use latest version of `memcached_store` gem into Shopify production, we had to rollback as we were getting lots of
`*** exception_class=Memcached::NotFound exception_message="Memcached::NotFound"`

This was mainly happening on `get_multi` calls.

### Solution
This PR is aiming to fix that problem:
- Update `memcached` dependency gem, as the new one handles NotFound errors better.
- Update `memcached`, because `log_exception` is only defined there.
- Rescue errors on `get_multi` method.

Also adding `NotFound` regression tests.

review @boourns @camilo @jstorimer @BlakeMesdag @dylanahsmith