Strategy to send annotations to back end

The current strategy of sending annotations to back end is done by sending over the whole data pack. This is quite a large network overhead. It is sufficient to send back the diff only. There are two possible approaches:

1. Sending the actual diff between the two packs.
2. Sending the annotation action, and apply the actions in the back end.

Start by tracing how annotation data packs are currently sent from the client to the back end; the issue names no files, tests, or entry points. Compare the two proposed strategies and define the request, application, and verification points before choosing an approach; done means annotations synchronize without sending the whole pack.

Written by the indexing model from the issue text.