Make `Arena` be a refcounted object.
Solves a bunch of issues: our stack right now needs a very complicated dance between transport and surface to destroy a call, but with this scheme we can just hold a ref to what we need in each place and everything works out.
Removes some `ifdef`'d out code that had been sitting dormant for a year or two also -- I'd left it in as a hedge against it being maybe a bad idea, but it looks like it's not needed.
Closes#36758
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36758 from ctiller:arena-counting d1b672fe30
PiperOrigin-RevId: 638767768
53c42e9dae