chore: move rdb save related logic into auxillary file (#1769)

1. No logic was changed during refactoring.
2. Flipped the flag to run regression tests for now own with zset_tree=on

Signed-off-by: Roman Gershman <[email protected]>

Author: romange

src/server/CMakeLists.txt

@@ -21,6 +21,7 @@ add_library(dragonfly_lib channel_store.cc command_registry.cc
             list_family.cc main_service.cc memory_cmd.cc rdb_load.cc rdb_save.cc replica.cc
             protocol_client.cc
             snapshot.cc script_mgr.cc server_family.cc malloc_stats.cc
+            detail/save_stages_controller.cc
             set_family.cc stream_family.cc string_family.cc
             zset_family.cc version.cc bitops_family.cc container_utils.cc io_utils.cc
             serializer_commons.cc journal/serializer.cc journal/executor.cc journal/streamer.cc