Introduce _inter and _intra arrangement variants

[frankmcsherry]

This is a rework of #687 which means to demonstrate how one can use timely's FrontierInterest to reduce the volume of scheduling that hits various operators. It reworks TraceAgent as two types TraceIntra and TraceInter, the former more efficient to schedule but not shareable across dataflows, and the latter busier to schedule and shareable across dataflows. The distinction is that the latter needs to be able to mirror the frontier into the other dataflow explicitly, as progress tracking does not span dataflows, and to do this it needs to be continually rescheduled as its frontiers evolve.

At the moment, there are mixed defaults of TraceIntra from arrangements with _inter variants to produce TraceInter versions, and TraceInter from reduce_trace which probably wants the same treatment as the arrangement functions.

There is an additional question about which other operators should gain FrontierInterest::IfCapability.

1. reduce_trace seems like a good candidate. It is very analogous to arrange_core.
2. join_traces would be great, but there is a catch: it uses its frontiers to relax logical compaction in the other inputs. This is a non-problem for reduce_trace, because in the absence of further inputs (which prompt scheduling) there is nothing to do. But join_traces could have one frontier advance and the other not, and it could hypothetically unlock further compaction. I'm not certain it is a problem, by analogy to reduce_trace: if there's no data from either of them, what compaction is there to perform? But it is a bit tenuous.
3. half_join also needs some thought. It also relaxes frontiers, and it does receive batch inputs from the trace (for scheduling), even though it ignores them. So potentially fine for the same reasons.

Each of these are subtle though, and trace compaction is something I've gotten wrong before.