Adding compression to terms dict from SortedSet/Sorted DocValues

Elasticsearch keyword field uses SortedSet DocValues. In our applications, “keyword” is the most frequently used field type.
LUCENE-7081 has done prefix-compression for docvalues terms dict. We can do better by replacing prefix-compression with LZ4. In one of our application, the dvd files were ~41% smaller with this change(from 1.95 GB to 1.15 GB).
I've done simple tests based on the real application data, comparing the write/merge time cost, and the on-disk *.dvd file size(after merge into 1 segment).

This feature is only for the high-cardinality fields. 
I'm doing the benchmark test based on luceneutil. Will attach the report and patch after the test.