Left boundary fix

Hey,
we introduced a bugfix for the Issue 451 a couple of weeks ago. This fixed that the option "connectSeparatedPoints" was not working for the left and right edges of charts with multiple series.
However this fix led to inconsistent boundaries behavior. A part of the fix was to choose different boundaries for each series to be able to draw the edge points for all of the series. The getLeftBoundary_ method however does not take the setIndex in account and returns the left boundary of the first set found. This leads to problems when highlighting points etc.

This bug fix gives the getLeftBoundary method the parameter "setIdx" and now makes it possible to get the row offset individually for each set.
I have adapted all calls of getLeftBoundary resulting in a couple of performance optimizations since due to the change that the points are now generated and maintained in dygraphs itself, we can read the row directly from the point object itself.
I also added a test that fails in the current master branch and works in this pull request.