Change BlockBasedTableOptions.format_version default to 2

Summary: BlockBasedTableOptions.format_version = 2 uses better encoding format. Now it's the time to make it default.

Test Plan: Run all existing tests.

Reviewers: igor, yhchiang, anthony, kradhakrishnan, andrewkr, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: MarkCallaghan, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D54879

HISTORY.md

@@ -1,5 +1,7 @@
 # Rocksdb Change Log
 ## Unreleased
+### Public API Changes
+* Change default of BlockBasedTableOptions.format_version to 2. It means default DB created by 4.6 or up cannot be opened by RocksDB version 3.9 or earlier.
 ### New Features
 * Add CompactionPri::kMinOverlappingRatio, a compaction picking mode friendly to write amplification.
 * Deprecate Iterator::IsKeyPinned() and replace it with Iterator::GetProperty() with prop_name="rocksdb.iterator.is.key.pinned"

include/rocksdb/table.h

@@ -166,7 +166,7 @@ struct BlockBasedTableOptions {
   // this.
   // This option only affects newly written tables. When reading exising tables,
   // the information about version is read from the footer.
-  uint32_t format_version = 0;
+  uint32_t format_version = 2;
 };
 
 // Table Properties that are specific to block-based table properties.