Reduce heap allocations in Parse

InterpretationScoringFeatures.createRawRecord may have been causing too many
unnecessary heap allocations so replace with Option and set to None.

One side effect is that whereas earlier an interpretation always had scores set
(even if empty), it will now be set only if non-empty. This is fine since the
thrift definition has scores declared as optional so no consumers should be
relying on it always being set.

Auditors: blackmad, nsanch