Storage service uses group0 internally, but group0 is create long after
storage service is initialized and passed to it using ss::set_group0()
function. But what it means is that during shutdown group0 is destroyed
before ss::stop() is called and thus storage service is left with a
dangling reference. Fix it by introducing a function that cancels all
group0 operations and waits for background fibers to complete. For that
we need separate abort source for group0 operation which the patch also
introduces.
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