move MDM types to micromdm/micromdm (#341)

Previously the github.com/micromdm/mdm library was used, but the code there was a bit naive and had a few issues as it grew to support the full library of MDM Commands. 

All the request types have been moved to `package mdm`.

platform/command/internal/commandproto/command.proto

@@ -5,8 +5,9 @@ package commandproto;
 message Event {
        	string id = 1;
        	int64 time = 2;
-        Payload payload = 3;
+        Payload payload = 3; // unused
         string device_udid = 4;
+        bytes payload_bytes = 5;
 }
 
 message Payload {