Initialize m_time_start, add m_size increment

Initialize m_time_start, add m_size increment

The m_time_start should be initialized, so that the time measurement
does not overflow sooner then 49 days after the start. Additionally the
m_time_start is reset in clear() method for the same reason.

The m_size was not adjusted upon creating a new measurement. This means
that no measurement but the last is was saved. This is now fixed.