libstore: turn builder output processing into event loop

this removes the rather janky did-you-mean-async poll loop we had so
far. sadly kj does not play well with pty file descriptors, so we do
have to add our own async input stream that does not eat pty EIO and
turns it into an exception. that's still a *lot* better than the old
code, and using a real even loop makes everything else easier later.

Change-Id: Idd7e0428c59758602cc530bcad224cd2fed4c15e

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