update操作buffer到DocumentsWriterDeleteQueue里，flush时处理deletes.
DocumentsWriterDeleteQueue使用global DeleteSlice和DWPT DeleteSlice存储deletes。
DWPT DeleteSlice
用来更新DWPT绑定的unFlushed segment中docID小于docIdUpTo的docs.
buffer
IndexWriter.updateDocuments更新docs时，update term转化成delete TermNode.
DWPT updateDocuments时，consumer.processDocument之前, 记录docsInRamBefore作为delete docIdUpTo.
finishDocuments时, 将deleteNode加入DWPT DeleteSlice和global DeleteSlice.
applyDeletes
DWPT flush该segment时，传入该segment BufferedUpdates，构造private segment的FrozenBufferedUpdates。
FreqProxTermsWriter.applyDeletes中读取term postings，处理pendingUpdates到小于docIDUpTo的docs。liveDocs中对应的bit设为0。

global DeleteSlice
用来更新flushed segments.
buffer
IndexWriter.deleteDocuments by Terms, deleteDocuments by Queries, updateDocValues时，
TermArrayNode，QueryArrayNode, DocValuesUpdatesNode直接add到global DeleteSlice.
IndexWriter.updateDocuments的delete TermNode既加入DWPT DeleteSlice，也加入global DeleteSlice.
global DeleteSlice中deleteNode的docIdUpTo为MAX INT.
buffer过大，或prepareFlush时，将globalBufferedUpdates构造global FrozenBufferedUpdates。
applyDeletes
IndexWriter flush segment后, applyAllDeletesAndUpdates时，
通过FrozenBufferedUpdates.applyTermDeletes，FrozenBufferedUpdates.applyQueryDeletes，处理global FrozenBufferedUpdates.
FrozenBufferedUpdates.applyTermDeletes
delete by terms, 直接删除包含exact terms的docs.
遍历segments，直接读取term的postings. 遍历postings删除doc。
FrozenBufferedUpdates.applyQueryDeletes
delete by queries, 走query流程，PhraseQuery等会analyze, normalize.
TermQuery情况下，等同于applyTermDeletes。
FrozenBufferedUpdates.

参考：
lucene8.7.0