Ensure per-row limit never equates to more than 10 seconds of data

[jeremyestein]

Currently each row is hard coded to have no more than 3000 data points in it. This is 10 seconds at 300Hz, but 60 seconds at 50Hz. If we generally aim for Emap to be no more than 10-20 seconds behind, this figure needs to be adjusted based on the sampling frequency for each stream.