Add two more optimization improvements to HISTORY (#6679)

Summary:
Although these optimizations are not user facing, still feel it's valuable to call out in HISTORY.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6679

Test Plan: no need

Reviewed By: zhichao-cao

Differential Revision: D20945916

Pulled By: cheng-chang

fbshipit-source-id: f3e790c07f3bcc4a8a74246c4fa232800ddd4438

HISTORY.md

@@ -27,6 +27,8 @@
 
 ### Performance Improvements
 * In CompactRange, for levels starting from 0, if the level does not have any file with any key falling in the specified range, the level is skipped. So instead of always compacting from level 0, the compaction starts from the first level with keys in the specified range until the last such level.
+* Reduced memory copy when reading sst footer and blobdb in direct IO mode.
+* When restarting a database with large numbers of sst files, large amount of CPU time is spent on getting logical block size of the sst files, which slows down the starting progress, this inefficiency is optimized away with an internal cache for the logical block sizes.
 
 ### New Features
 * Basic support for user timestamp in iterator. Seek/SeekToFirst/Next and lower/upper bounds are supported. Reverse iteration is not supported. Merge is not considered.