The ResponseWriter would truncate the last series if the byte size of
the points frames exceeded the writeSize constant, causing a Flush to
occur and the cumulative ResponseWriter.sz to reset to zero. Because
ResponseWriter.sz was not incremented for each frame, it remained at
zero, which resulted in the final Flush short circuiting.
This commit implements the Size method for the cursors.Array types
to be used to estimate the size of frame. This is in place of calling
the Protocol Buffer `Size` function, which can be very expensive.
3ca751cfd6