Tested and fixed utility for truncating history data.

HistoryUtil.truncateHistory now actually works as intended. Care must be
taken not to interfere with the history data while it is being
truncated. This must be ensured externally.

A reference counter (AtomicInteger) was also added to ChartData to be
able to see how many users the ChartData has and to wait until it no
longer has any users. This is required to wait until all chart editors
have detached themselves from the history data.

refs #7622

Change-Id: I35863f5258da2cae193b3086500b5514c4699518