Handle the following improbable but risky race conditions in the range deleter service:

• The onStepDown hook may be invoked even if no step-up had previously happened in case of rollback (even when rollbacking happening on a SECONDARY node):
  • Relax this invariant.
  • Do not assume the executor exists as the onStepUpComplete hook may not have been previously invoked.

• As the recovery of range deleter tasks is only spawned on step-up but may complete later, it's not assumable that the node is still primary when the recovery finishes:
  • Acquire the mutex and double check that the node is still primary before transitioning to "up" state