It seems that currently most of the time is spent iterating through all
the cells to get the optimal row height. We can easily optimize by using
mdds::flat_segment_tree.

Now we don't need to iterate through all cells and instead can work on
whole blocks.

Change-Id: Id9a0686490b0e91ad6552b428c38a9f6635a7938
Reviewed-on: https://gerrit.libreoffice.org/27856Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>