iris: implement inter-context busy-tracking

This is a backport from the patch from !14505 (merged) (which fixes issues #5731 (closed) and #5812 (closed)) for the stable branch.

A recent commit to Iris (a90a1f15) added some significant conflicts and so I'm having to rework my original fix, which will result in not-so-trivial conflicts to backport. There is some important interaction in the order where we do things so I figured it's better to not leave the rebase to the backporters (specifically, this function reads from and later writes to deps->write_syncobjs[batch_idx] and a backport may accidentally invert the order, rendering the fix useless, just like my first rebase attempt).

MR !14505 (merged) will soon be updated with the rebased patch.