Return ordinals from MultiOrdinals.MultiDocs

MultiOrdinals.MultiDocs returned 'null' ordinals which caused
a NPE if the field was single valued and would allow a significantly
smaller in memory representation than single packed int ordinals.

Closes #3470
Added random sort test for dense and sparse fields.

This test triggers a MultiDoc / MultiOrds in-memory representation
even if the field is not multivalued

Relates to #3470
termvectors: fix null pointer exception if field has no term vectors

Retrieving termvectors for a document that does not have the requested field
caused a null pointer exception. Same for documents if the field has no term vectors,
for example, because the field only contains "?".
Now, an empty response is returned.

Closes #3471
Fix inconsistent usage of ScriptScoreFunction in FiltersFunctionScoreQuery

This commit fixes inconsistencies in `function_score` and `filters_function_score`
using scripts, see issue #3464

The method 'ScoreFunction.factor(docId)' is removed completely, since the name
suggests that this method actually computes a factor which was not the case.
Multiplying the computed score is now handled by 'FiltersFunctionScoreQuery'
and 'FunctionScoreQuery' and not implicitely performed in
'ScoreFunction.factor(docId, subQueryScore)' as was the case for 'BoostScoreFunction'
and 'DecayScoreFunctions'.

This commit also fixes the explain function for FiltersFunctionScoreQuery. Here,
the influence of the maxBoost was never printed.

Closes #3464