Make meta_start_ and meta_end_ private.

Start addressing #50 (closed)

For now I made the two members private and added functions for accessing them.

The issue with removing meta_end_ completely are:

• Streaming mode uses getTimeStampOfLastSample which needs meta_end_. I am not sure if this could be replaced by using the final element of window_metadata_ or not?
• updateMetaWindowAccordingToSampleWindow - in case window_metadata_ is empty meta_end_ is used to search for the ref_meta. I'm not sure where this would be used, but as is I don't see how to keep the functionality without also keeping meta_end_.

Removing meta_start_:

• Here the meta_start_ is used before window_metadata_ is populated. With a comment that in some cases it meta_start_ may be overwritten so I assume we also can't always use window_metadata_.

Since you have a better understanding of the whole behaviour maybe you have an idea what we can and can't do.