This one works similar to `serializer<optional<T>>` and will be
later needed for serializing `raft::append_request`, which has
a field containing `lw_shared_ptr`.
Users to be warned, though: this code assumes that the pointer
is never null. This is done to mirror the serialize implementation
for `lw_shared_ptr:s` in the messaging_service.cc, which is
subject to being deleted in favor of the impl in the
`serializer_impl.hh`.
Signed-off-by: Pavel Solodovnikov <pa.solodovnikov@scylladb.com>
