Add a new releaser method, which then is invoked to release
buffers on failure.

It is a bit contrived how we try not break external
implementations when adding a new default implementation
to PositionedReadable: the releaser will be lost unless
they do so.

Change-Id: Ie2f95a89cfa79d3fdf33c1cc58f63e4ca00c6207

Trying to implement resilience in the s3a input stream;
buffers of incomplete reads are now released on failures.

Change-Id: I4b23c6a2ff417687fd19c1e8e9feef6cf733c031

Trying to implement resilience in the s3a input stream;
buffers of incomplete reads are now released on failures.

Change-Id: I851779383f8d120fe42bc97c5e6ae533c5fe2e23

Now: how do I pull this out so it can be used with the prefetch
stream?

Change-Id: I98e350ab3b28d9adc7b458dee4ceee56a0562f17