Keep builds on the bus under their module, so no lookup reads the bucket whole (issue 1121, ADR 0427)

Milestone 5 moved 8,599 build records onto the bus under bare build ids, each
carrying its module's whole manifest. Every build lookup then read the whole
bucket, and a delivery plan's send did that per module per machine while it
held the plans and the machine's declaration: the send of build-agent never
finished, and every other act and the status waited behind it.

Builds now live under <module>.build.<id> (requests under req.<id>, id.<id>
names a build's module), with pointer keys newest, worked, held,
commit.<sha> and source.<reuse key> that move only to a newer build and are
read as hints, repaired when wrong. Manifests are kept once in the same
bucket under manifest.sha256-<hex>, so no grant the running bus lacks is
needed. The old keys are laid out once at start, idempotently, and left
unread. The calls a plan look makes on the silent clocks while it holds the
plans are bounded, since the vendored kv.Keys ignores its context.

internal/chain/keys_test.go

@@ -0,0 +1,38 @@
+package chain
+
+import (
+	"context"
+	"testing"
+)
+
+func TestSubjectsMatchAsTheBusFiltersThem(t *testing.T) {
+	for _, c := range []struct {
+		pattern, key string
+		want         bool
+	}{
+		{"alpha.>", "alpha.build-1", true}, {"alpha.>", "alpha.commit.c1", true}, {"alpha.>", "beta.build-1", false},
+		{"alpha.>", "alpha", false}, {"*.newest", "alpha.newest", true}, {"*.newest", "alpha.commit.newest", false},
+		{"*", "build-1", true}, {"*", "alpha.build-1", false}, {">", "a.b.c", true}, {"req.*", "req.build-1", true},
+	} {
+		if got := SubjectMatches(c.pattern, c.key); got != c.want {
+			t.Errorf("SubjectMatches(%q, %q) = %v", c.pattern, c.key, got)
+		}
+	}
+}
+
+func TestOnlyTheKeysOfAFilterAreWatched(t *testing.T) {
+	ctx := context.Background()
+	c := InMemory(nil)
+	for _, k := range []string{"alpha.build.b1", "alpha.build.b2", "beta.build.b3", "alpha.newest"} {
+		if _, err := c.Builds.CreateRaw(ctx, k, []byte(`{"id":"x"}`)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	got, err := c.Builds.FilteredRaw(ctx, ModuleFilter("alpha"))
+	if err != nil || len(got) != 2 {
+		t.Fatalf("alpha.> gave %v: %v", got, err)
+	}
+	if !IsRecordKey("alpha.build.b1") || IsRecordKey("alpha.newest") || IsRecordKey("alpha.commit.c") || IsRecordKey("manifest.build.x") || IsRecordKey("req.b1") || IsRecordKey("id.b1") {
+		t.Error("IsRecordKey tells a record from a pointer wrongly")
+	}
+}