Improve RareClusterStateIT (#105390)

Today this test suite relies on being able to cancel an in-flight
publication after it's reached a committed state. This is questionable,
and also a little flaky in the presence of the desired balance allocator
which may introduce a short delay before enqueuing the cluster state
update that performs the reconciliation step.

This commit removes the questionable meddling with the internals of
`Coordinator` and instead just blocks the cluster state updates at the
transport layer to achieve the same effect.

Closes #102947

Author: DaveCTurner