Re-add index_per_partition but as deprecated

Summary:
index_per_partition should have deprecated deprecated instead of being removed. It is causing backward compatibility issues.
Closes https://github.com/facebook/rocksdb/pull/2173

Differential Revision: D4910947

Pulled By: maysamyabandeh

fbshipit-source-id: 5c52939381847d232ede6866606f67f2b4b857ae

options/options_helper.h

@@ -600,6 +600,9 @@ static std::unordered_map<std::string, OptionTypeInfo>
         {"index_block_restart_interval",
          {offsetof(struct BlockBasedTableOptions, index_block_restart_interval),
           OptionType::kInt, OptionVerificationType::kNormal, false, 0}},
+        {"index_per_partition",
+         {0, OptionType::kUInt64T, OptionVerificationType::kDeprecated, false,
+          0}},
         {"metadata_block_size",
          {offsetof(struct BlockBasedTableOptions, metadata_block_size),
           OptionType::kUInt64T, OptionVerificationType::kNormal, false, 0}},