Rename QueryEntityForAllSlots to SolrQueryExecutor
Move SingleSolrQueryExecutor code into SolrQueryExecutor.
Reorganize package structure to keep related code grouped together.
Convert SolrQueryExecutor to class.
Rename KbpSolrQuery and QueryBuilder to SolrQuery and SolrQueryBuilder for consistency.
Change query code to do one slot at a time.
Modify CandidateSet, remove CandidateExtraction and SlotCandidateSet, create Answer