Move multi-sample logic to C++ 

The C++ implementation is ugly and slow, but should be correct.

This moves multi-sample logic from `loom.query.MultiSampleProtobufServer` to `loom::QueryServer` in `src/query_server.cc` https://github.com/priorknowledge/loom/pull/46/files#diff-3e16722759b678b24dabd87bbc910c79R1