fix partial sstable writes

[thomasjungblut]

In some failure scenarios we might have inconsistent data and index
files. This adds seeking support to recordio writers, which can be used
to rewind partially written data records.

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todo testing:

• if the last sstable partial record was never overwritten by a new record, the iterator might still return it (or at least fail the EOF assertion)
• more boundary testing for the recordio aspect of the buffering