Simplify the parent cleaning in the prune_tree() mechanism

Simplify the parent cleaning in the prune_tree() mechanism

Instead of juggling with node locks in a cycle, cleanup the node we are
just pruning and send any the parent that's also subject to the pruning
to the prune tree via normal way (e.g. enqueue pruning on the parent).

This simplifies the code and also spreads the pruning load across more
event loop ticks which is better for lock contention as less things run
in a tight loop.

(cherry picked from commit 0b32d323e07d2468111aa725b36e72b0538e0521)