Add getMany() to Cache/DatabaseStore.php to save on DB requests.

[tylercubell]

Cache/RetrievesMultipleKeys.php has a many() function that retrieves multiple items from the cache by key. It looks like each key lookup requires a separate database query (see Cache/DatabaseStore.php).

If Cache/DatabaseStore.php were to have a complementary getMany() function that used whereIn() instead of where() (among other supporting changes), it would only require one database request.