Backtracking is coarser than necessary

[johntyree]

Analysis of the AssignmentSet changelogs seems to indicate that backtracking in the sat-solver is far too aggressive. What typically happens is a whole suite of packages are suggested by the policy and assumed True until finally a conflict is found. Then everything is unwound back to the beginning and almost all of the same packages are re-suggested by the policy. This repeats many times over the course of the search.

To fix this, we need to be able to backtrack to the previous suggested package and not back to the root of the search tree. I think it's supposed to work this way already, so closing this will either require a bug fix or a good explanation for why it's happening.