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.go

@@ -0,0 +1,66 @@
+// Package contextutil contains functions for working with contexts.
+package contextutil
+
+import (
+	"context"
+	"time"
+)
+
+type mergedCtx struct {
+	ctx1, ctx2 context.Context
+
+	doneCtx    context.Context
+	doneCancel context.CancelFunc
+}
+
+// Merge merges two contexts into a single context.
+func Merge(ctx1, ctx2 context.Context) (context.Context, context.CancelFunc) {
+	mc := &mergedCtx{
+		ctx1: ctx1,
+		ctx2: ctx2,
+	}
+	mc.doneCtx, mc.doneCancel = context.WithCancel(context.Background())
+	go func() {
+		select {
+		case <-ctx1.Done():
+		case <-ctx2.Done():
+		case <-mc.doneCtx.Done():
+		}
+		mc.doneCancel()
+	}()
+	return mc, mc.doneCancel
+}
+
+func (mc *mergedCtx) Deadline() (deadline time.Time, ok bool) {
+	if deadline, ok = mc.ctx1.Deadline(); ok {
+		return deadline, ok
+	}
+	if deadline, ok = mc.ctx2.Deadline(); ok {
+		return deadline, ok
+	}
+	return mc.doneCtx.Deadline()
+}
+
+func (mc *mergedCtx) Done() <-chan struct{} {
+	return mc.doneCtx.Done()
+}
+
+func (mc *mergedCtx) Err() error {
+	if err := mc.ctx1.Err(); err != nil {
+		return mc.ctx1.Err()
+	}
+	if err := mc.ctx2.Err(); err != nil {
+		return mc.ctx2.Err()
+	}
+	return mc.doneCtx.Err()
+}
+
+func (mc *mergedCtx) Value(key interface{}) interface{} {
+	if value := mc.ctx1.Value(key); value != nil {
+		return value
+	}
+	if value := mc.ctx2.Value(key); value != nil {
+		return value
+	}
+	return mc.doneCtx.Value(key)
+}