test-osd issus about nentropy

[wuxingyi]

1. the oom problem is triggered by badger. When closing a badger instance, a flush is conducted, badger creates a very big buffer(96MB) for each instance(which is hard coded), when closing 100 instances, 9GB memory usage triggered the oom.

// NewTableBuilder makes a new TableBuilder.
func NewTableBuilder() *Builder {
	return &Builder{
		keyBuf:     newBuffer(32 << 20),
		buf:        newBuffer(64 << 20),
		prevOffset: math.MaxUint32, // Used for the first element!
	}
}

I already made a pull request :dgraph-io/badger#281, which will save much memory because we can use much smaller table size.

2. message will output to log file soon.