* grandpa: fix creation of justification ancestry

we would reject commits that have precommits targeting blocks lower than the
commit target. when there is an equivocation (or if it the commit is not
minimal) it is possible to have such precommits and we should assume that they
are the round base.

* grandpa: bump to 0.16.0

* grandpa: add test for justification with equivocation

* grandpa: fix failing test