Make blocking pool non-static and use for thread pool (#1678)

Previously, support for `blocking` was done through a static `POOL` that
would spawn threads on demand. While this made the pool accessible at
all times, it made it hard to configure, and it was impossible to keep
multiple blocking pools.

This patch changes `blocking` to instead use a "default" global like the
ones used for timers, executors, and the like. There is now
`blocking::with_pool`, which is used by both thread-pool workers and the
current-thread runtime to ensure that a pool is available to tasks.

This patch also changes `ThreadPool` to spawn its worker threads on the
blocking pool rather than as free-standing threads. This is in
preparation for the coming in-place blocking work.

One downside of this change is that thread names are no longer
"semantic". All threads are named by the pool name, and individual
threads are not (currently) given names with numerical suffixes like
before.

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