Update release version to 6.9.0 (#6610)

Summary:
Update release version to 6.9.0
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6610

Test Plan: no code change

Reviewed By: riversand963

Differential Revision: D20741094

Pulled By: zhichao-cao

fbshipit-source-id: 80a9e9ea8d164b6923112352d36fcbc1be85c034

HISTORY.md

@@ -1,5 +1,5 @@
 # Rocksdb Change Log
-## Unreleased
+## 6.9.0 (03/29/2020)
 ### Public API Change
 * Fix spelling so that API now has correctly spelled transaction state name `COMMITTED`, while the old misspelled `COMMITED` is still available as an alias.
 * Updated default format_version in BlockBasedTableOptions from 2 to 4. SST files generated with the new default can be read by RocksDB versions 5.16 and newer, and use more efficient encoding of keys in index blocks.

include/rocksdb/version.h

@@ -5,7 +5,7 @@
 #pragma once
 
 #define ROCKSDB_MAJOR 6
-#define ROCKSDB_MINOR 8
+#define ROCKSDB_MINOR 9
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with