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/queue/internal/devicecommandproto/device_command.proto

@@ -0,0 +1,25 @@
+syntax = "proto3";
+
+package devicecommandproto;
+
+message Command {
+    string uuid = 1;
+    bytes payload = 2;
+
+    int64 created_at = 3;
+    int64 last_sent_at = 4;
+    int64 acknowledged = 5;
+
+    int64 times_sent = 6;
+
+    string last_status = 7;
+    bytes failure_message = 8;
+}
+
+message DeviceCommand {
+    string device_udid = 1;
+    repeated Command commands = 2;
+    repeated Command completed = 3;
+    repeated Command failed = 4;
+    repeated Command not_now = 5;
+}