* Convert downloader Observable to flow
Uses `runInterruptible` to turn the blocking call to `queue.take()`
into a cancellable call.
Flow collection is ended by cancelling the scope in `recycle`. This
means the `HttpPageLoader` can't be reused after calling `recycle`,
but this was true with the `Observable` as well.)
* Convert load Observables to suspending function
Inlining the Observables allows for some simplification of the error
handling. Behavior should be otherwise identical.
* Convert cleanup Completable to coroutine
Uses global `launchIO`, not ideal but similar to previous behavior.
Can't be scheduled on the local `scope` as this runs after `scope` is
cancelled.
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