Reimplement zlib streaming.

This time with less bugs. It turned out that the crash was due to a
synchronisation issue between the pending reads and the actual shard
polling mechanism.

Essentially the pending reads would be cancelled via a simple bool but
there would still be a pass left and thus we would have a single
pending read left before or after running the polling mechanism and
this would cause a race condition.

Now the pending read mechanism is properly waited for before returning
control back to the caller.

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