Fix schema emptying issue from watch mode

[benjie]

Description

Fixes #1761

There was two bugs in the introspection plugin:

1. the cache was being invalidated too early, which meant that multiple database events in quick succession would incorrectly reuse previous caches
2. the cache also cached the "announcements" of introspection, meaning when the cache is used the schema building wouldn't even see the database changes!

I've refactored the code to separate the side effects from the introspection so that they can be cached separately (side effects to state, introspection to permanent cache) and I've now tracked whether the introspection results are "dirty" or not - if another event arrives before introspection of the DB occurs then we'll mark the introspection as dirty to ensure that it isn't reused.

Performance impact

None known.

Security impact

None known.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[changeset-bot]

🦋 Changeset detected

Latest commit: cbf3820

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
graphile-build-pg	Patch
postgraphile	Patch
graphile-utils	Patch
pgl	Patch
graphile	Patch

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