Remove another "#arrivals" reference

Was missed in 9e45f9710a
This won't be needed once the async is corrected

This made a second updateSolariBoard() call 2 seconds after page load to correct for the data being one update behind.
Don't assume 8 rows

When clearing the board or searching for the next due time.
Have NextDue() take a class name, not status code

This will help with scoping issues later with the new_board = EMPTY_ROW check.
Reorder where code runs with the async getJSON() call

Currently, the board redraws with data from the last ajax post before it has a chance to use results from the current ajax.  So it's always drawing data that is a minute old.  This moves the code into the callback function so it has the current data.
Blank time handling

One of the comments mentions next due should read from "the row for which time info is available, which may not be from the first"  But in testing, this edit is needed to handle blank times correctly.
clearBoard() should reset the next due clock as well