ISPN-3718  Select which protobuf fields to index

Use protobuf schema doc comment annotations similar with the Hibernate Search annotations @Indexed and @Field.

https://issues.jboss.org/browse/ISPN-3718

depends on https://github.com/infinispan/protostream/pull/9 to be integrated first.
I like ```stored=true``` better.

Getting rid of @Indexed is possible but not trivial because then we can no longer have message level metadata directly produced by the schema parser (see IndexingMetadata). So we'll need to rethink the way we scan for field level metadata and aggregate it ourselves and store it somewhere per message type. Given the complexity involved I'd rather have two annotations for both message and field level. In future the message level annotation might have an attribute for the index name or other things similar to Hibernate Search.