Fix bug in TensorCircularBuffer::Clear()

[xiaomengy]

This PR fixed the crash issue for TensorCircularBuffer::Clear() and also add a Reset() function for CircularBuffers. The differences of Clear() and Reset() are listed below. The related issue is #85

• Clear() will only make the TensorCircularBuffer's size == 0, but will not reset the internal status such as initialized_ and schema_. Clear() will be more efficient for adding the same data schema multiple times.
• Reset() will call Clear() and then reset all of the internal status such as initialized_ and schema_.