Fix rebalance for zero weighted devices.

If we set device's weight to zero, currently balance will be set
special value(999.99) until zero weighted device return all
its partitions. So we cannot check balance has changed.
Thus we need to check balance or last_balance is special value.

Change-Id: I5b7db8b8e48db0c4771c51a764bda689869817d5
Fixes: bug #1171731
Adds chunk_size to CompressingFileReader.

Before when iterating using this object, the call to the underlying file object
was made without a chunk_size.  This is bad because this would cause the entire
contents of the file to be read.

Change-Id: I9956e5d2d693a6260252fff331d4f78f70179a6c
Merge "Fix rebalance for zero weighted devices."
Merge "Adds chunk_size to CompressingFileReader."