Consistency model aspects

[maackle]

PR summary

Based on #1974, this updates the consistency model to deal primarily with Aspects, rather than entries, for holding and publishing.

This will allow us to do things like iterating over the holding list of a freshly started instance, sending a HoldAspect consistency signal for each one, to let hachiko know about what's been held before even starting the network.

testing/benchmarking notes

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followups

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changelog

• if this is a code change that effects some consumer (e.g. zome developers) of holochain core, then it has been added to our between-release changelog with the format

- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

documentation

• this code has been documented according to our docs checklist