Apply and commit: Write changes to the datastore after broadcasting them to clients

[GoogleCodeExporter]

The wave server should be responsible for two steps when a delta is submitted 
by a user:
First, apply the delta and immediately sent a onAck message to client.
Second, commit the delta: write the delta to persistent storage and then sent a 
onCommit message to client.

The Design Overview page said:
    "Walkaround writes changes to the datastore before broadcasting them to other clients; this avoids the need for complicated rollback protocols."

To process the rollback, I guess most of the work will be done in 
GenericOperationChannel. When rollback occurs, the channel should try to resend 
the missing operations, so that the server side can silently ignore the 
corresponding commit fails.

The more complicated two step process is really a killer feature, please 
consider it. 

Original issue reported on code.google.com by i...@goodow.com on 25 Oct 2012 at 10:28