Update HISTORY.md for 6.4.0 (#5714)

Summary:
Update HISTORY.md by removing a feature from "Unreleased" to 6.4.0 after cherry-picking related commits to 6.4.fb branch.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5714

Differential Revision: D16865334

Pulled By: riversand963

fbshipit-source-id: f17ede905a1dfbbcdf98806ca398c618cf54748a

HISTORY.md

@@ -1,8 +1,5 @@
 # Rocksdb Change Log
 ## Unreleased
-### New Features
-* Support loading custom objects in unit tests. In the affected unit tests, RocksDB will create custom Env objects based on environment variable TEST_ENV_URI. Users need to make sure custom object types are properly registered. For example, a static library should expose a `RegisterCustomObjects` function. By linking the unit test binary with the static library, the unit test can execute this function.
-
 ### Bug Fixes
 * Fixed a number of data races in BlobDB.
 
@@ -29,6 +26,7 @@
 * Compression dictionary blocks are now prefetched and pinned in the cache (based on the customer's settings) the same way as index and filter blocks.
 * Added DBOptions::log_readahead_size which specifies the number of bytes to prefetch when reading the log. This is mostly useful for reading a remotely located log, as it can save the number of round-trips. If 0 (default), then the prefetching is disabled.
 * Added new option in SizeApproximationOptions used with DB::GetApproximateSizes. When approximating the files total size that is used to store a keys range, allow approximation with an error margin of up to total_files_size * files_size_error_margin. This allows to take some shortcuts in files size approximation, resulting in better performance, while guaranteeing the resulting error is within a reasonable margin.
+* Support loading custom objects in unit tests. In the affected unit tests, RocksDB will create custom Env objects based on environment variable TEST_ENV_URI. Users need to make sure custom object types are properly registered. For example, a static library should expose a `RegisterCustomObjects` function. By linking the unit test binary with the static library, the unit test can execute this function.
 
 ### Performance Improvements
 * Reduce iterator key comparision for upper/lower bound check.