When the scheduler tries to find a placement for a new allocation, it iterates
over a subset of nodes. For each node, we populate a `NetworkIndex` bitmap with
the ports of all existing allocations and any other allocations already proposed
as part of this same evaluation via its `SetAllocs` method. Then we make an
"ask" of the `NetworkIndex` in `AssignPorts` for any ports we need and receive
an "offer" in return. The offer will include both static ports and any dynamic
port assignments.

The `AssignPorts` method was written to support group networks, and it shares
code that selects dynamic ports with the original `AssignTaskNetwork`
code. `AssignTaskNetwork` can request multiple ports from the bitmap at a
time. But `AssignPorts` requests them one at a time and does not account for
possible collisions, and doesn't return an error in that case.

What happens next varies:

1. If the scheduler doesn't place the allocation on that node, the port
   conflict is thrown away and there's no problem.
2. If the node is picked and this is the only allocation (or last allocation),
   the plan applier will reject the plan when it calls `SetAllocs`, as we'd expect.
3. If the node is picked and there are additional allocations in the same eval
   that iterate over the same node, their call to `SetAllocs` will detect the
   impossible state and the node will be rejected. This can have the puzzling
   behavior where a second task group for the job without any networking at all
   can hit a port collision error!

It looks like this bug has existed since we implemented group networks, but
there are several factors that add up to making the issue rare for many users
yet frustratingly frequent for others:

* You're more likely to hit this bug the more tightly packed your range for
  dynamic ports is. With 12000 ports in the range by default, many clusters can
  avoid this for a long time.
* You're more likely to hit case (3) for jobs with lots of allocations or if a
  scheduler has to iterate over a large number of nodes, such as with system jobs,
  jobs with `spread` blocks, or (sometimes) jobs using `unique` constraints.

For unlucky combinations of these factors, it's possible that case (3) happens
repeatedly, preventing scheduling of a given job until a client state
change (ex. restarting the agent so all its allocations are rescheduled
elsewhere) re-opens the range of dynamic ports available.

This changeset:

* Fixes the bug by accounting for collisions in dynamic port selection in
  `AssignPorts`.
* Adds test coverage for `AssignPorts`, expands coverage of this case for the
  deprecated `AssignTaskNetwork`, and tightens the dynamic port range in a
  scheduler test for spread scheduling to more easily detect this kind of problem
  in the future.
* Adds a `String()` method to `Bitmap` so that any future "screaming" log lines
  have a human-readable list of used ports.