Fixes #3214: Multimap get & remove should return correct collection type

Fixes https://github.com/hazelcast/hazelcast/issues/3214

- Currently multimap get & remove always return list type. With this fix, they should return correct collection type depending on the configuration of multimap's value collection type.
Again a complex expression. The following reads a lot better:

```
Data item = serializationService.toData(record.getObject())
collection.add(item);
```