Coalesce partitions above and below network coalesce

[gabotechs]

Rework of #285.

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Coalesces partitions before and after a network boundary, instead of just after it:

This:

        │     CoalescePartitionsExec
        │       [Stage 1] => NetworkCoalesceExec: output_partitions=6, input_tasks=3
        └──────────────────────────────────────────────────
          ┌───── Stage 1 ── Tasks: t0:[p0..p1] t1:[p2..p3] t2:[p4..p5] 
          │ AggregateExec: mode=Partial, gby=[], aggr=[count(Int64(1))]
          │   ProjectionExec: expr=[]

Becomes this:

        │     CoalescePartitionsExec
        │       [Stage 1] => NetworkCoalesceExec: output_partitions=3, input_tasks=3
        └──────────────────────────────────────────────────
          ┌───── Stage 1 ── Tasks: t0:[p0] t1:[p1] t2:[p2] 
          │ CoalescePartitionsExec
          │   AggregateExec: mode=Partial, gby=[], aggr=[count(Int64(1))]
          │     ProjectionExec: expr=[]

This allows us to:

1. Simplify the network communication between the boundaries
2. Do some early work in a distributed manner for when coalescing implies ordering with SortMergePreservingExec

[gene-bordegaray]

This looks good, one small nit

[gabotechs]

Benchmarking this now...

[gabotechs]

I'm getting pretty bad results with the benchmarks for this one... I'm still double checking if it's something wrong with the benchmarks themselves