std::rt: Reduce MessageQueue contention

It's not a huge win but it does reduce the amount of time spent
contesting the message queue when the schedulers are under load
std::rt: Remove metrics for perf

These aren't used for anything at the moment and cause some TLS hits
on some perf-critical code paths. Will need to put better thought into
it in the future.
std: Convert the runtime TLS key to a Rust global to avoid FFI