Add a new variant of the reply to the direct_fd_ping which specifies
whether the local group0 is alive or not, and start actively using it.
There is no need to introduce a cluster feature. Due to how our
serialization framework works, nodes which do not recognize the new
variant will treat it as the existing std::monostate. The std::monostate
means "the node and group0 is alive"; nodes before the changes in this
commit would send a std::monostate anyway, so this is completely
transparent for the old nodes.
a8ee4d543a