Implement queue for watchGather/watchSchema/etc

[benjie]

Need to test this before merging.

[changeset-bot]

🦋 Changeset detected

Latest commit: 6a4e3b9

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

This PR includes changesets to release 15 packages

Name	Type
graphile-build	Patch
postgraphile	Patch
grafserv	Patch
@dataplan/pg	Patch
pgl	Patch
grafast	Patch
graphile-build-pg	Patch
graphile-utils	Patch
graphile	Patch
@grafserv/persisted	Patch
@localrepo/grafast-bench	Patch
@dataplan/json	Patch
ruru-components	Patch
@localrepo/grafast-website	Patch
graphile-export	Patch

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[benjie]

Tested via:

(for I in `seq 1 1000`; do echo 'create table my_table(id serial primary key, name text); drop table my_table;'; done) | psql watchtest

With

let schemaCounter = 0;
const DelaySetPresetPlugin = {
  name: "DelaySetPresetPlugin",
  grafserv: {
    middleware: {
      setPreset(next, event) {
        return sleep(1000).then(next);
      },
    },
  },

  schema: {
    hooks: {
      finalize(schema, build) {
        console.log(`Produced ${++schemaCounter}th schema`);
        return schema;
      },
    },
  },
};

Running PostGraphile hooked up to this schema created 24 schemas total, even though the above should have triggered 2000 schema build notifications. The 1 second delay was clearly respected.