[DF] Remove usage of stacks which made several components re-entrant

this commits removes a substantial amount of code which made the
execution of core operations reentrant. Reentrancy is not a requirement
any more since the work spawned via TThreadExecutors is isolated, which
means that a task that submits subtasks, e.g. via a parallel_for, and
is idle waiting those subtasks to finish cannot pull in a work item
which was spawned by any of its parents.
Concretely, it means that while processing a cluster of entries in a
slot, it cannot happen that the processing is interrupted to start
processing a new cluster.