Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Fixes #1885

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

This commit factors out the common task queue logic in the basic
scheduler runtime and the `LocalSet` struct in `tokio::task`. This is
because as more work was done on the `LocalSet`, it has gotten closer
and closer to the basic scheduler in behavior, and factoring out the
shared code reduces the risk of errors caused by `LocalSet` not doing
something that the basic scheduler does.

In particular, I noticed the basic scheduler has a flag that indicates
the remote queue has been closed, which is set when dropping the
scheduler. This prevents tasks from being added after the scheduler has
started shutting down, stopping a potential task leak. Rather than
duplicating this code in `LocalSet`, I thought it was probably better to
factor it out into a shared type.

There are a few cases where there are small differences in behavior,
though, so there is still a need for separate types implemented _using_
the new `Queues` struct. However, it should cover most of the identical
code.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

this should hopefully make it clearer that these are not used by the
threadpool.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>