There might be cases where we have created a panc, but have not inserted it
into the linked list yet. Instead of asserting we have inserted it, simply
conditionally update the head of the list.
Instead of having a separate "busy" flag and pending scry request state,
we now have a single "busy" unit that, if there is a pending request,
contains details about the pending request. In the ames case, this is
simply %ames. But in the scry case, it contains all the details we need
to cancel the request, timer, or both when needed.
Additionally, we now make sure to always cancel the scry timeout timer
whenever we get a scry response.
Previously, when the scry timeout fired and we retried the request with
ames, we would always start over from the very first request (ie,
fetching the ""manifest"" containing the files we need to download).
This behavior is not correct in the case where we had already received
that initial response, and were now fetching individual files. So here,
if that's the case, we simply call into +work after marking this peer as
having known-broken scry, which will resume work as appropriate.
(The actual bug here that broke the fallback behavior entirely was the
busy flag not getting unset. We now clean it up properly.)
We also move setting of the sad timer into +retry-with-ames, instead of
doing it at each individual callsite. (In fact, one of the callsites was
missing this behavior.)
This accounts for a possible race condition where ames expects a
response, but regresses into the larval state. Upon receiving the
$sign on +take, we would remain stuck as a larva. Now we check
that we have enough information to re-evolve and then start a
/larval timer to begin draining the queue.
When +ke-abet-gone gets called, we're going to remove this keen
entirely. +ke-set-wake does a whole song-and-dance and may even set a
new timer. So instead, we simply call +ke-rest, if we have a known
outstanding timer.
