Be more principled about when blocking is ok (#2410)

This enables `block_in_place` to be used in more contexts. Specifically,
it allows you to block whenever you are off the tokio runtime (like if
you are not using tokio, are in a `spawn_blocking` closure, etc.), and
in the threaded scheduler's `block_on`. Blocking in `LocalSet` and the
basic scheduler's` block_on` is still disallowed.

Fixes #2327.
Fixes #2393.