Add configuration option to turn off ignoring NATIVE cache when fetch by key

[yuriyz]

Currently when cache type is NATIVE and we use it by key then we skip cache and call couchbase/ldap directly since it doesn't make sense to duplicate same entry in couchbase/ldap (under gluu_cache or ou=cache). However in some cases it can be useful to make it configurable and monitor performance and usages, therefore this ticket is to introduce configuration property to turn off this optimization.