Avoid error: invalid application of 'sizeof' to an incomplete type 'rocksdb:xxx' when compiling with C++-23

[mpatou-openai]

When trying to compile with c++-23 I'm getting errors like:

/usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/unique_ptr.h:91:16: error: invalid application of 'sizeof' to an incomplete type 'rocksdb::FragmentedRangeTombstoneList'
   91 |         static_assert(sizeof(_Tp)>0,
      |                       ^~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/unique_ptr.h:398:4: note: in instantiation of member function 'std::default_delete<rocksdb::FragmentedRangeTombstoneList>::operator()' requested here
  398 |           get_deleter()(std::move(__ptr));
      |           ^
cpp/third-party/rocksdb-cloud/db/range_tombstone_fragmenter.h:25:62: note: in instantiation of member function 'std::unique_ptr<rocksdb::FragmentedRangeTombstoneList>::~unique_ptr' requested here
   25 |   std::unique_ptr<FragmentedRangeTombstoneList> tombstones = nullptr;
      |                                                              ^
cpp/third-party/rocksdb-cloud/db/range_tombstone_fragmenter.h:20:8: note: forward declaration of 'rocksdb::FragmentedRangeTombstoneList'
   20 | struct FragmentedRangeTombstoneList;
      |        ^

With this fix declaration are reargenged to avoid this and when not possible we give explicit constructor/destructors and move them to a different place (ie. not in the headers).