organize essential APIs into platform, workflow and pkg folders (#337)

Add more logic to the way code is organized.

	/pkg -- library code not directly connected to micromdm
	/mdm -- packages meant for the services devices interract with. The MDM protocol.
	/dep -- DEP API and related packages.
	/platform -- Core APIs the server provides. Commands API, Devices API, queue, pubsub etc.
	/workflow -- Packages/API that build on top of platform. Today that's the webhook package.
		     Depending on what ends up here, the workflow folder might become its own repository.

platform/command/service.go

@@ -0,0 +1,67 @@
+// Package command provides utilities for creating MDM Payloads.
+package command
+
+import (
+	"time"
+
+	"github.com/go-kit/kit/log"
+	"github.com/go-kit/kit/metrics"
+	"github.com/micromdm/mdm"
+	"golang.org/x/net/context"
+)
+
+type Service interface {
+	NewCommand(context.Context, *mdm.CommandRequest) (*mdm.Payload, error)
+}
+
+// Middleware describes a service (as opposed to endpoint) middleware.
+type Middleware func(Service) Service
+
+// ServiceLoggingMiddleware returns a service middleware that logs the
+// parameters and result of each method invocation.
+func ServiceLoggingMiddleware(logger log.Logger) Middleware {
+	return func(next Service) Service {
+		return serviceLoggingMiddleware{
+			logger: logger,
+			next:   next,
+		}
+	}
+}
+
+func (mw serviceLoggingMiddleware) NewCommand(ctx context.Context, req *mdm.CommandRequest) (p *mdm.Payload, err error) {
+	defer func(begin time.Time) {
+		mw.logger.Log(
+			"method", "NewCommand",
+			"error", err,
+			"took", time.Since(begin),
+		)
+	}(time.Now())
+	return mw.next.NewCommand(ctx, req)
+}
+
+type serviceLoggingMiddleware struct {
+	logger log.Logger
+	next   Service
+}
+
+// ServiceInstrumentingMiddleware returns a service middleware that tracks the
+// number of payloads created by the service.
+func ServiceInstrumentingMiddleware(p metrics.Counter) Middleware {
+	return func(next Service) Service {
+		return serviceInstrumentingMiddleware{
+			payloads: p,
+			next:     next,
+		}
+	}
+}
+
+type serviceInstrumentingMiddleware struct {
+	payloads metrics.Counter
+	next     Service
+}
+
+func (mw serviceInstrumentingMiddleware) NewCommand(ctx context.Context, req *mdm.CommandRequest) (*mdm.Payload, error) {
+	p, err := mw.next.NewCommand(ctx, req)
+	mw.payloads.Add(1)
+	return p, err
+}