Implement sparse LiveDocs to reduce memory by up to 8x (#15413)

Author: salvatorecampagna

lucene/CHANGES.txt

@@ -225,6 +225,8 @@ Optimizations
 
 * GITHUB#14963: Bypass HNSW graph building for tiny segments. (Shubham Chaudhary, Ben Trent)
 
+* GITHUB#13084: Implement sparse LiveDocs format to reduce memory usage by up to 8x for segments with <=1% deletions. (Salvatore Campagna)
+
 * GITHUB#15397: NumericComparator: immediately check whether a segment is competitive with the recorded bottom (Martijn van Groningen)
 
 # GITHUB#15303: Speed up NumericUtils#{add,subtract} by operating on integers instead of bytes. (Kaival Parikh)