Split up parallel sections to allow further refactors

[Benjamin Went (MetBenjaminWent)]

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Issue: #571
Umbrella: #106

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[Benjamin Went (MetBenjaminWent)]

At commit 3

[Benjamin Went (MetBenjaminWent)]

At standard seg size 16, at the top for bl.explicit it doesn't make much difference (note lack of kernels in data)

[Benjamin Went (MetBenjaminWent)]

Post some further work to add nowaits and swap to static blocking, no real positive change, it's still worse off.

However, one bonus is that at the top level caliper, nothing really changes as the exchanges are dominating the cost still.