Revert "Products.CMFActivity.ActivityTool: Improve behaviour on single-node instances."

Also revert related fixup commits:
  "CMFActivity: fixup do not loop on tic if the node is the distribution node"
  "Products.ERP5Type.tests: Follow-up on ActivityTool.tic signature change."

While the original commit did improve the specific workload it was
designed to improve, it turned out to degrade too much intensive activity
workloads, like initial ERP5 site creation and tests (which, for the
purposes of this change, are the same as a single-zope instance).
Given how easy it is to get a multi-Zope instance, which would solve the
original issue and also provide the better performance necessary anyway
for an instance managing a non-trivial amount of documents, I choose to
revert this change.

I am not reverting several loosely-related changes I applied, which
rather fix real bugs uncovered by the different activity execution
scheme this change provided, especially by letting tests'
"stop_condition" callback being executed a lot more often between
activities, uncovering missing dependencies and unrealistic test
expectations, whose fixes should be beneficial independently from the
reverted code.

This reverts commit 4dfafbc9.
This reverts commit 4eb26017.
This reverts commit 041642d0.