Change handling of RunStatus packets

Refs #6661
Add code to validate requested start time for live listener

Added code to LoadLiveData.cpp to validate the time string for the
requested start time.  Also use the epoch time for the "start from
now" option (instead of using the current time).  The epoch time gets
converted to 0 when passed to SMSD and SMSD handles that a little more
efficiently than passing it the current time.

Refs #6661
Merge branch 'feature/6661_LiveListenerDataStart' into develop