Update HISTORY.md for blob cache (#10328)

Summary:
Update HISTORY.md for blob cache.  Implementation can be found from Github issue #10156 (or Github PRs #10155, #10178, #10225, #10198, and #10272).

Pull Request resolved: #10328

Reviewed By: riversand963

Differential Revision: D37732514

Pulled By: gangliao

fbshipit-source-id: 4c942a41c07914bfc8db56a0d3cf4d3e53d5963f

HISTORY.md

@@ -4,6 +4,10 @@
 * Mempurge option flag `experimental_mempurge_threshold` is now a ColumnFamilyOptions and can now be dynamically configured using `SetOptions()`.
 * Support backward iteration when `ReadOptions::iter_start_ts` is set.
 * Provide support for ReadOptions.async_io with direct_io to improve Seek latency by using async IO to parallelize child iterator seek and doing asynchronous prefetching on sequential scans.
+* Added support for blob caching in order to cache frequently used blobs for BlobDB. 
+ * User can configure the new ColumnFamilyOptions `blob_cache` to enable/disable blob caching.
+ * Either sharing the backend cache with the block cache or using a completely separate cache is supported. 
+ * A new abstraction interface called `BlobSource` for blob read logic gives all users access to blobs, whether they are in the blob cache, secondary cache, or (remote) storage. Blobs can be potentially read both while handling user reads (`Get`, `MultiGet`, or iterator) and during compaction (while dealing with compaction filters, Merges, or garbage collection) but eventually all blob reads go through `Version::GetBlob` or, for MultiGet, `Version::MultiGetBlob` (and then get dispatched to the interface -- `BlobSource`). 
 
 ### Public API changes
 * Add metadata related structs and functions in C API, including