change the writelock queue to a deque and allow merges to jump to the front of the line.  in general, against an aggressive said of workers writing new segments, this isn't enough.  the header segment list grows steadily and gets far too big.
progress on automerge.  cascade merge asyncs was more trouble than it's worth.  automerge just checks levels 0 through 7.  should be configurable.  if desperate (TBD), do the merge synchronously.  on the aggressive writers test, the header gets really big before the first merge completes and then the desperate check can kick in, and then it gradually shrinks back down.
tryMerge now returns a function instead of an async wrapper around that same function.  also, for the moment, all automerges are happening synchronously rather than in the background.
experimenting with automerge immediate vs delayed
notes