Dispatcher::none() and NoSubscriber have no effect when set per-thread

[g2p]

The dispatch logic ignores NoSubscriber when it is the thread-local subscriber.

Having the default per-thread subscriber forward to the global subscriber makes sense, but should use a purpose-built sentinel value. Otherwise NoSubscriber isn't usable and doesn't work as documented (with_subscriber, set_subscriber, with_dispatcher, set_dispatcher all work at the thread level and won't have the desired effect).

Discovered while working on tokio-rs/console#314.

[g2p]

Ah, reported in #1999 and fixed in #2001.

(I was mystified that NoSubscriber didn't work ever since tokio-console 0.1)