Partial fix for millisecond timestamps where more than 32,767 data points
were stored in a single RowSeq. Larry Reeder found this bug where an assertion
was thrown because the short index rolled over. Changing it to an int will help
but only allows up to 2.1M data points in a row whereas the schema allows 4M.
TODO - figure out an efficient means of keeping the full 4M bytes in memory
with quick access for iteration.

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>