* Fix dense y-axis labels in heatmap using tsbuckets

This change emulates the non-tsbuckets Y-axis tick count by making
the y-axis label formatter treat a dense set as a sparse set.

fixes #11342

* have d3 do the work

* Update public/app/plugins/panel/heatmap/rendering.ts

Co-authored-by: Alexander Zobnin <alexanderzobnin@gmail.com>

Co-authored-by: Alexander Zobnin <alexanderzobnin@gmail.com>