Fixes for node-restarting in each scheduling
Fixes for node-restarting in each scheduling

* node2collection should only be populated when the collection is
  added.  This way collection_is_completed can be kept track of
  correctly.

* The _removed2pending map needs to have the entire node as key
  because it is needed to look up the collection of the gateway which
  died in node2collection to check the collection of the replacement
  node is identical.

* init_distribute() needs to actually keep track of the started nodes
  otherwise it's (re)scheduling gets all confused and will re-schedule
  too often.
nicoddemus/pytest-xdist,RonnyPfannschmidt/pytest-xdist,pytest-dev/pytest-xdist,lukas-bednar/pytest-xdist-convert