Client connections are not restored after server restarts

I found some bug that I've introduced myself with:
https://issues.apache.org/jira/browse/WAVE-288
and
http://codereview.waveprotocol.org/612001

The problem is that ServerRpcProvider.Connection is not persisted or
restored in server restarts, so we get errors like:

Session[XyEIEO-HQIEm1DPqZZGf]: Exception thrown by
SocketIOInbound.onMessage()
java.lang.IllegalArgumentException: Auth token invalid

and also we lost editions etc, because the webclient seems is correct connected.

As a workaround, please remove _session/* between WIAB restarts.

A patch is comming.