Delete all documents in the index.
This method will drop all buffered documents and will remove all segments from the index. This change will not be visible until a {@link #commit()} has been called. This methodcan be rolled back using {@link #rollback()}.
NOTE: this method is much faster than using deleteDocuments( new MatchAllDocsQuery() ).
NOTE: this method will forcefully abort all merges in progress. If other threads are running {@link #forceMerge}, {@link #addIndexes(IndexReader[])} or{@link #forceMergeDeletes} methods, they may receive{@link MergePolicy.MergeAbortedException}s.