Ensures that reused buffers of the batch importer are properly zeroed out

Batch importer makes use of a special window pool that reuses some of the
 buffers it requires. In some cases, it is possible for previous uses
 of a buffer to leave state in them, resulting in subsequent passes
 over the store to write out content that might cause problems
 later on. In particular, during store migration of a store that
 has large not in use record spans in it, it is possible that
 the target store would have contents in the same offesets that
 has not properly zeroed out records.
This fix ensures that when the buffer is going to be reused it
 is zeroed out first.