lifetime logic: use agree instead of dec_agree

If you are happy with this, we can merge this and https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/22

Cc @robbertkrebbers @jjourdan

I'm happy with that. Please merge.

added 3 commits

• 02ec7298...741c78cf - 2 commits from branch master
• 84913cf3 - lifetime logic: use agree instead of dec_agree

Compare with previous version

mentioned in commit 3ec09815

merged