Client append retry on ErrNotEnoughSpace

- Client retries entire append op if ErrNotEnoughSpace is reported.
- Add TestAppend2 for testing appending data exceeds chunk size.
- Bugfix: Chunk servers pads a chunk if the current offset of the chunk
  plus the length of the data is greater then chunk size, instead of
  padding a chunk when the current offset is greater then three fourth
  of the chunk size.