Support for indexes on custom attribute types (backport)

Issue #3857
Fix for indexes on custom object types that do not resolve to any AttributeType but implement Comparable.
Key change is at IndexImpl.saveEntryIndex (lines 80-85). Even if attribute type is null we can still use an index.
Test: QueryIndexTest.testResultsReturned_whenCustomAttributeIndexed
Split up QueryBasicTest into multiple files.

Backport of https://github.com/hazelcast/hazelcast/pull/4053