[SPARK-12869] Implemented an improved version of the toIndexedRowMatrix

Hi guys,

I've implemented an improved version of the `toIndexedRowMatrix` function on the `BlockMatrix`. I needed this for a project, but would like to share it with the rest of the community. In the case of dense matrices, it can increase performance up to 19 times:
https://github.com/Fokko/BlockMatrixToIndexedRowMatrix

If there are any questions or suggestions, please let me know. Keep up the good work! Cheers.

Author: Fokko Driesprong <f.driesprong@catawiki.nl>
Author: Fokko Driesprong <fokko@driesprongen.nl>

Closes #10839 from Fokko/master.