Fix a backward compatibility issue (#9456)

Summary:
Fix a backward compatibility issue caused by removing
`purge_redundant_kvs_while_flush`. Reserve the option internally.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9456

Test Plan: CI: https://app.circleci.com/pipelines/github/facebook/rocksdb/11122/workflows/b7bc0f35-1be8-432c-9292-79125e22ecc7/jobs/280595

Reviewed By: ajkr, ltamasi

Differential Revision: D33808474

Pulled By: jay-zhuang

fbshipit-source-id: 7c3b553bc8e85c8a560514e8e460a2dbaf25718d

examples/rocksdb_option_file_example.ini

@@ -103,6 +103,7 @@
   level0_stop_writes_trigger=20
   compression=kSnappyCompression
   level0_file_num_compaction_trigger=4
+  purge_redundant_kvs_while_flush=true
   max_write_buffer_size_to_maintain=0
   memtable_factory=SkipListFactory
   max_grandparent_overlap_factor=8

options/cf_options.cc

@@ -509,6 +509,9 @@ static std::unordered_map<std::string, OptionTypeInfo>
         {"compaction_measure_io_stats",
          {0, OptionType::kBoolean, OptionVerificationType::kDeprecated,
           OptionTypeFlags::kNone}},
+        {"purge_redundant_kvs_while_flush",
+         {0, OptionType::kBoolean, OptionVerificationType::kDeprecated,
+          OptionTypeFlags::kNone}},
         {"inplace_update_support",
          {offset_of(&ImmutableCFOptions::inplace_update_support),
           OptionType::kBoolean, OptionVerificationType::kNormal,

options/options_test.cc

@@ -93,6 +93,7 @@ TEST_F(OptionsTest, GetOptionsFromMapTest) {
       {"inplace_update_support", "true"},
       {"report_bg_io_stats", "true"},
       {"compaction_measure_io_stats", "false"},
+      {"purge_redundant_kvs_while_flush", "false"},
       {"inplace_update_num_locks", "25"},
       {"memtable_prefix_bloom_size_ratio", "0.26"},
       {"memtable_whole_key_filtering", "true"},
@@ -2255,6 +2256,7 @@ TEST_F(OptionsOldApiTest, GetOptionsFromMapTest) {
       {"inplace_update_support", "true"},
       {"report_bg_io_stats", "true"},
       {"compaction_measure_io_stats", "false"},
+      {"purge_redundant_kvs_while_flush", "false"},
       {"inplace_update_num_locks", "25"},
       {"memtable_prefix_bloom_size_ratio", "0.26"},
       {"memtable_whole_key_filtering", "true"},