Fix Pax build error in Rocky Linux 8

Downgrade the protobuf version requirement from 3.6.1 to 3.5.0 so that
we can use the default protobuf version 3.5.0 in Rocky Linux 8 and avoid
building protobuf separately.

Also, add an explicit `#include <atomic>` to fix compilation error
realted to missing `std::atomic` definition caused by gcc-10.x.

See: github.com//issues/1097

Author: gongxun0928

contrib/pax_storage/FindDependencies.cmake

@@ -1,6 +1,6 @@
 ## protobuf
 include(FindProtobuf)
-find_package(Protobuf 3.6.1 REQUIRED)
+find_package(Protobuf 3.5.0 REQUIRED)
 
 # ztsd
 # in our image snapshot, zstd is managed using pkg-config, so so the pkg-config method is used first here

contrib/pax_storage/src/cpp/storage/filter/pax_sparse_filter.h

@@ -28,6 +28,7 @@
 #pragma once
 #include "comm/cbdb_api.h"
 
+#include <atomic>
 #include <map>
 #include <memory>
 #include <utility>
@@ -189,4 +190,5 @@ class PaxSparseFilter final {
   bool allow_fallback_to_pg_;
 };  // class PaxSparseFilter
 
-}  // namespace pax
+}  // namespace pax
+