VBucketFileWriter should close the chunk before closing the snapshot file

Currently the snapshot file is closed before closing this data chunk. This is invalid because in the case where the (most likely Rift) data writer is asynchronous (such as it is/will be for cloud) it's possible for the last persisted seqno to be ahead of the data that we have on disk.

If we commit the data that we have on disk (which will be a blocking operation) then we persist the snapshot file we know that we have all the data.