MoveTreeCompact
Records that the ops identified by droppedDots have been garbage-collected from the move-log.
Broadcast to peers so they can apply the same trim via MovableTree.applyCompact. Receiving the same MoveTreeCompact twice is idempotent.
The droppedDots are re-emitted by MovableTree.causalDots in the compacted tree so that the Quilter's contiguous delivered frontier has no holes after GC removes raw ops from the log.