"prune" materializer task

[sandreae]

Introduces a "prune" materialization task which:

• is issued whenever a "reduce" task completes when the input was a DocumentId (this is a new current view)
• the "prune" task looks at the updated document and deletes any rows from the document_views and document_view_fields table where: they are not the current view AND they are not targeted from a pinned relation of any other document

I also had to make some changes to prune_document_views method which was introduced onto the store in a previous PR cos it was not doing what it's supposed to....

📋 Checklist

• [x] Add tests that cover your changes
• [x] Add this PR to the Unreleased section in CHANGELOG.md
• [x] Link this PR to any issues it closes
• [x] New files contain a SPDX license header

[codecov[bot]]

Codecov Report

Patch coverage: 97.55% and project coverage change: +0.99% :tada:

Comparison is base (81cae95) 91.20% compared to head (0a93f68) 92.20%.

:exclamation: Current head 0a93f68 differs from pull request most recent head 8c7938f. Consider uploading reports for the commit 8c7938f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #496 +/- ## =============================================== + Coverage 91.20% 92.20% +0.99% =============================================== Files 101 102 +1 Lines 16393 16644 +251 =============================================== + Hits 14952 15347 +395 + Misses 1441 1297 -144 ``` | [Files Changed](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda) | Coverage Δ | | |---|---|---| | [aquadoggo/src/materializer/tasks/reduce.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9tYXRlcmlhbGl6ZXIvdGFza3MvcmVkdWNlLnJz) | `95.04% <86.20%> (-0.24%)` | :arrow_down: | | [aquadoggo/src/test\_utils/node.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy90ZXN0X3V0aWxzL25vZGUucnM=) | `92.51% <96.61%> (-2.32%)` | :arrow_down: | | [aquadoggo/src/materializer/tasks/prune.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9tYXRlcmlhbGl6ZXIvdGFza3MvcHJ1bmUucnM=) | `98.07% <98.07%> (ø)` | | | [aquadoggo/src/db/stores/document.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9kYi9zdG9yZXMvZG9jdW1lbnQucnM=) | `98.74% <99.00%> (+12.93%)` | :arrow_up: | | [aquadoggo/src/materializer/service.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9tYXRlcmlhbGl6ZXIvc2VydmljZS5ycw==) | `94.76% <100.00%> (+0.01%)` | :arrow_up: | | [aquadoggo/src/materializer/tasks/dependency.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9tYXRlcmlhbGl6ZXIvdGFza3MvZGVwZW5kZW5jeS5ycw==) | `97.54% <100.00%> (+<0.01%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/p2panda/aquadoggo/pull/496/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda)

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[sandreae]

This work is being continued in a new branch here: https://github.com/p2panda/aquadoggo/pull/500