Introduce custom Externalizers in the Query module: BooleanQuery serialization support
Add Externalizer for TermQuery
Introduce Externalizer for Lucene Term
Introduce Externalizer for Lucene Sort and SortField
Include ClusteredTopDocs and TopDocs Externalizer
Externalizer for ScoreDoc
Add Externalizer for TopFieldDocs
Include an Externalizer integration test focusing on Lucene Term
Improve code for ClusteredQueryTest
Review PriorityQueue handling