Summary: Add ZSTD compression type. The same way as adding LZ4.

Test Plan: run all tests. Generate files in db_bench. Make sure reads succeed. But the SST files cannot be opened in older versions. Also some other adhoc tests.

Reviewers: rven, anthony, IslamAbdelRahman, kradhakrishnan, igor

Reviewed By: igor

Subscribers: MarkCallaghan, maykov, yoshinorim, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D45747