refactor: Remove specialized 0rtt rpc calls

[rklaehn]

Actually, they are not yet removed but just marked as deprecated.

You can now just call the normal ones, and they will work for 0rtt connections as well. This has no measurable overhead. See benchmarks below.

We also rename the somewhat internal fn zero_rtt_accepted in RemoteConnection to zero_rtt_rejected. This is so the code reads nicer for the case where there isn't any 0rtt to begin with.

This is a pretty obscure part of the API, but nevertheless is a breaking change.

if zero_rtt_rejected {
  // resend
}

---

This branch:

Remote bench
RPC seq 15_590 rps
RPC par 80_014 rps

main:

Remote bench
RPC seq 15_596 rps
RPC par 79_363 rps