postgres: databroker storage backend (#3370)

* wip

* storage: add filtering to SyncLatest

* don't increment the record version, so intermediate changes are requested

* databroker: add support for query filtering

* fill server and record version

* postgres: databroker storage backend

* wip

* serialize puts

* add test

* skip tests for macos

* add test

* return error from protojson

* set data

* exclude postgres from cover tests

pkg/contextutil/contextutil_test.go

@@ -0,0 +1,34 @@
+package contextutil
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMerge(t *testing.T) {
+	t.Run("value", func(t *testing.T) {
+		ctx1 := context.WithValue(context.Background(), "key1", "value1") //nolint
+		ctx2 := context.WithValue(context.Background(), "key2", "value2") //nolint
+		ctx3, _ := Merge(ctx1, ctx2)
+		assert.Equal(t, "value1", ctx3.Value("key1"))
+		assert.Equal(t, "value2", ctx3.Value("key2"))
+	})
+	t.Run("cancel", func(t *testing.T) {
+		ctx1, cancel1 := context.WithCancel(context.Background())
+		defer cancel1()
+		ctx2, cancel2 := context.WithCancel(context.Background())
+		ctx3, _ := Merge(ctx1, ctx2)
+		cancel2()
+		assert.Eventually(t, func() bool {
+			select {
+			case <-ctx3.Done():
+				return true
+			default:
+				return false
+			}
+		}, time.Second, time.Millisecond*100)
+	})
+}