The server's fixes for a refused batch walked every deleted layer's whole subtree again and
compared each document-level fix with all the earlier ones, so a batch deleting a deep chain took
quadratic time on the hub thread. Each subtree walk now stops at a layer already restored, and
document-level fixes are looked up and sent once per target. The fixes are unchanged.
A refused batch deleting a 20,000-deep chain plus 40,000 comment puts: 27.2 s before, 0.14 s after
(debug build).
Fixes#1063
BFlat
08173cc51e