U1db implementation

In u1db's `HTTPApp` code, the string `sync-from` is used to identify the HTTP request from the source to the target as a sync request. The sync request is mapped to the `SyncResource` class, which is able to respond to each HTTP action of such requests.

The complete URL path for the sync requests are `/<target-replica-uid>/sync-from/<source-replica-uid>`. A `GET` to that path gets the sync info from the target. A `POST` to that path is the actual `sync_exchange()` (the exchange of documents) between source and target; the body of the request contains the documents sent from source to target and the response of the request contains the documents sent from target to source. A `PUT` to that path updates the target with the source metadata (`generation` and `transaction_id`) after the document exchange has finished (that is needed when the source receives documents from the target and thus has its generation increased).

I could not find any occurrence of `sync-to` in u1db trunk, can you point me to it?

Also, in U1DB the sync attempt is always issued by the source, so in a sense it is always a "sync-from".

There is no problem if we want to change the semantics here, but we should be explicit about it.

I think I did not understand how this `SyncType` message would be useful. Can you givbe me an example of the usage?