enable multiple serializers at once in the server

[irmen]

It would be nice if it was possible to enable more than one active serializer in the server. This requires a change in the wire protocol: it needs to encode the serializer use.

[irmen]

I've started implementing this. The write protocol indeed does change (also impacts Pyrolite). A new configuration option will appear that allows you to select the set of serializers that are accepted on the server. (replying to a client is always done with the serializer that the client is using)