Update release version to 6.16 (#7782)

Summary:
Update release version to 6.8

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7782

Reviewed By: siying

Differential Revision: D25648579

Pulled By: jay-zhuang

fbshipit-source-id: c536d606868b95c5fb2ae8f19c17eb259d67bc51

HISTORY.md

 # Rocksdb Change Log
-## Unreleased
+## 6.16.0 (12/18/2020)
 ### Behavior Changes
 * Attempting to write a merge operand without explicitly configuring `merge_operator` now fails immediately, causing the DB to enter read-only mode. Previously, failure was deferred until the `merge_operator` was needed by a user read or a background operation.
 
@@ -7,8 +7,6 @@
 * Truncated WALs ending in incomplete records can no longer produce gaps in the recovered data when `WALRecoveryMode::kPointInTimeRecovery` is used. Gaps are still possible when WALs are truncated exactly on record boundaries; for complete protection, users should enable `track_and_verify_wals_in_manifest`.
 * Fix a bug where compressed blocks read by MultiGet are not inserted into the compressed block cache when use_direct_reads = true.
 * Fixed the issue of full scanning on obsolete files when there are too many outstanding compactions with ConcurrentTaskLimiter enabled.
-
-### Bug Fixes
 * Fixed the logic of populating native data structure for `read_amp_bytes_per_bit` during OPTIONS file parsing on big-endian architecture. Without this fix, original code introduced in PR7659, when running on big-endian machine, can mistakenly store read_amp_bytes_per_bit (an uint32) in little endian format. Future access to `read_amp_bytes_per_bit` will give wrong values. Little endian architecture is not affected.
 * Fixed prefix extractor with timestamp issues.
 * Fixed a bug in atomic flush: in two-phase commit mode, the minimum WAL log number to keep is incorrect.

include/rocksdb/version.h

@@ -5,7 +5,7 @@
 #pragma once
 
 #define ROCKSDB_MAJOR 6
-#define ROCKSDB_MINOR 15
+#define ROCKSDB_MINOR 16
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with